There can't be a "definitive" translation, because the pseudo-Latin precedes the popularity of the English. That second link you offer is actually good. Henry Beard offers *Noli nothi permittere te terere.*

Personally, I could see a few tweaks. Instead of *nothi*, I'd subsitute it with *spurios* (needs the accusative). Also, *te terere* sounds clumsy; I'd be tempted to use *attere* instead.

Alternatively, to be less literal, I'd opt for: *Ne terant te spurii.* Typically for "let not" Latin employs *ne* + the subjunctive. Compare that with e.g. Cicero's *De Legibus* 2.16.41, *donis impii **ne** placare **audeant** deos* ("let not the wicked dare [to try] to appease the gods with gifts").