At first I also thought this was a basic question, but I think it is actually not that easy to answer.
Let us first look at your suggestions. I am afraid the answer to your question: “Am I off by miles?” is yes. You are on the right path with the three words mors, malus and memoria but beyond that, frankly, the sentences look like a leisurely but somewhat aimless walk through the declension table ;)
A bad memory is mala memoria. Actually I suspect that rather means “bad memory” in the sense that someone cannot remember things well. In any event I would prefer a more expressive word like tristis memoria, which means a sad/unhappy/depressing memory. The plural is tristes memoriae.
Now my first impulse was: The English “death to X” should be expressed with the Latin dative, yielding:
Mors tristibus memoriis.
And I am pretty certain an ancient Roman would understand that, just as a medieval monk would. But it seems to me this is not the idiomatic way to express the idea in Latin; at least I could find no example older than the motto of an American air force unit.
A more typical Latin expression would be Perea(n)t X, which is, for example, found in the famous song Gaudeamus igitur:
Down with sadness,
Down with the haters.
So I would ultimately suggest this form:
Pereant tristes memoriae.
Literally: “May unhappy memories perish.” Like “perish” in English, the verb perire strongly suggests death.