There are numerous ways to express the idea of "kill off" without actually using those exact words. One could say "eradicate," "exterminate," or "annihilate" to convey the same meaning. Other options include "eliminate," "obliterate," or "destroy." In some contexts, the phrase "put an end to" could also serve as a synonym for "kill off." Additionally, phrases like "wipe out" or "stamp out" could be used to convey a similar sense of complete removal or destruction. Ultimately, the choice of phrase depends on the tone and context of the message being conveyed.