AHA! I did wonder why question was so obvious
Anyway, couple of possible solutions exists.
1) Node "AI:AISetCharacter" with character value "Hostage" will make AI ignored.
2) Node "AI:ChangeParameter" could be useful, as it allows changing the species of AI, thus causing it being treated as friendly and being ignored if species is same as with former enemies. Side effect is of course, depending of the set species, that some other AI will treat it hostile.