1. If you do this again, it will be the last time I [forgive, will forgive, am forgiving] you.
Any of the above will do. The pronoun "that" is more natural than "this," since the speaker is not associated with the other person's behavior:
1. If you do that again,...
2. This is the last time we [are, will be, will have been] together before you move to Tokyo. Let's make the most of it.
Any of the above will do, although the "will"-future seems more natural.