There are several ways of doing this, as you've seen. Obviously, the basic goal is to 'mark' the word in question in such a way that it can easily be understood to stand, in spite of its inflected form, outside the grammar of the sentence, without having to rely on (or rely exclusively on) a 'cheat' such as quotation marks. The use of something like verbum or vocabulum, or Joonas's vox, would do this.
But I want to expand on the option that Joel Derfner has mentioned: prefacing the word – or words – in question with the appropriate case form of the neuter singular demonstrative illud. Although Joel tentatively alluded to this in the context of contemporary spoken Latin, it's also quite common in Classical Latin. (That said, it doesn't work particularly well for your examples, because it assumes some previous or well-known instance of the words that you want to talk about.) By adding illud, you 'mark' what follows and effectively treat it as a neuter singular noun, even if nothing about it is neuter or singular or even a noun. Another neuter singular pronoun (e.g., id or hoc) would also work, but illud is what I'm most familiar with. Although modern texts will generaly print quotation marks around the word or words that are being marked in this way (unless they're Greek), it's the illud, of course, that does the actual work in the Latin text.
One example occurs in Cicero, Epistulae ad Atticum 6.1.11:
in Pompeio te spem omnem oti ponere non miror; ita res est, removendumque censeo
...I recommend that 'dissimulantem' must be removed.
Without the illud (and the helpful quotation marks that the editor has included), the clause could be read to mean only 'I recommend that the dissembling person must be removed.' So, although some grammatical sense could be made of this example, it wouldn't be a logical sense in context.
Next, there's Quintilian, Institutio oratoria 9.3.64, where the illud marks the otherwise-problematic finite verb edico. And since the illud effectively turns the finite verb form into a neuter nominative singular noun, it can itself be the subject of another finite verb form, convenit. In this example, without the illud, the sentence would make no grammatical sense at all as is.
quamvis enim pars [bello] posterior participio insistat, utrique convenit illud 'edico'.
...'edico' goes with both.
Earlier, in Inst. orat. 6.3.62, illud marks the finite verb form petis, and the neuter nominative singular noun that is thus formed is then not only used as the subject of another finite verb form, est, but also modified by a neuter singular adjective, ambiguum. Once again, the sentence would make no sense without the illud.
iungitur amphiboliae similitudo, ut a L. Galba, qui pilam neglegenter petenti 'sic' inquit 'petis tamquam Caesaris candidatus'. nam illud 'petis' ambiguum est, securitas similis.
...For 'petis' is ambiguous....
In Aulus Gellius, Noctes Atticae 18.9.10, illud is used to treat a Greek neuter plural noun as a Latin neuter singular noun:
sed etiam ipsum illud ἔπη, quod significat verba aut versus, non aliunde esse dictum tradunt quam ἀπὸ τοῦ ἕπεσθαι καὶ τοῦ εἰπεῖν. eadem ergo ratione antiqui nostri narrationes sermonesque 'insectiones' appellitauerunt.
But even ἔπη itself....
And illud can even be used in this way, not just with individual words, but with phrases or clauses, as in Cicero, Epistulae ad familiares 9.22.1:
quid est enim illud
'quae mulier una',
quid, inquam, est
'usurpat duplex cubile'?
For what is 'quae mulier una'? What, I say, is 'usurpat duplex cubile'?
In all the preceding examples, the illud – and therefore the word or words after it that are effectively treated as a neuter singular noun for grammatical purposes – is in either the nominative or accusative. This makes sense, since it's in these forms that the demonstrative is most recognizable as neuter, as brianpck has noted in a comment. Still, the same technique can also be used when the grammar of the sentence requires some other case.
In fact, the example that I'm most familiar with – the first example that really made me sit up and take notice of this technique – is Pliny the Younger, Epistulae 3.16.13, where the illud phrase is in the ablative case (ablative of comparison):
videnturne haec tibi maiora illo 'Paete, non dolet', ad quod per haec perventum est?
Do these words seem greater than 'Paete, non dolet'...?
In this example, the words after the form of illud are actually a famous quotation. So, although I've been omitting any explicit translation of the form of illud for all previous examples, treating it as just a marker, I could follow the practice of many translators here and in other similar examples, and translate it as 'that famous saying,' 'that well-known utterance,' or the like.
In Varro, De lingua latina 8.10, we find another ablative (of instrument/means), where the form of illud is strengthened by a form of idem:
sic quod dicimus in loquendo 'consul fuit Tullius et Antonius', eodem illo 'et' omnis binos consules colligare possumus, vel dicam amplius, omnia nomina, atque adeo etiam omnia verba, cum fulmentum ex una syllaba illud 'et' maneat unum.
...we can join together every set of two consuls by the same 'et'....
Then in Aulus Gellius, Noctes Atticae 1.21.4, we find a genitive:
sed enim cum Favorino Hygini commentarium legissem atque ei statim displicita esset insolentia et insuavitas illius 'sensu torquebit amaro', risit et: 'Iovem lapidem,' inquit 'quod sanctissimum iusiurandum habitum est, paratus ego iurare sum Vergilium hoc numquam scripsisse, sed Hyginum ego verum dicere arbitror....'
...the unusualness and unpleasantness of 'sensu torquebit amaro'...
And finally, in C. Caesius Bassus, De metris, we find a dative (with the adjective par):
hoc ipsum imminuendo paulatim tria haec metra quae rettuli manifestiora faciam: 'tu ne quaesieris quem mihi quem tibi': hoc par est illi 'Maecenas atavis edite regibus'.
...this is the same (metrically) as [that famous verse] 'Maecenas atavis edite regibus.'