I just wanted to add that there are other ways of stating a prohibition in Latin.
In addition to what I have written above, you could say "Noli ignoscere
," or "Cave ignoscas
." This second example I took straight from Allen and Greenough's New Latin Grammar and is attested in classical texts. http://books.google.com/books?id=8roAAA ... &ct=result
"Noli ignoscere" very literally says "be unwilling/refuse/wish not to forgive."
"Cave ignoscas" very literally says "take care/beware lest you forgive.
Of course, since you're getting a tattoo, you'll have to consider how 'natural' you want the Latin to read, versus how you want it to look. If you were to choose these you can rest assured that a Classics professor would approve, but you might not care about that. Although I don't think "Numquam ignosce" is the best translation, go with it if you couldn't bear looking at the other ones. I suppose what matters is that you like your own tattoo. I have a friend with a Latin tattoo which isn't entirely grammatical, and I haven't mentioned it because 1) it can't be undone and I don't want to cause needless unhappiness, and 2) my friend gets meaning out of it regardless of whether the noun and adjective agree in gender.