[zo <<sei] says that whatever is following it is not part of the actual statement. For example: [mi zi'opre sei jitfa] = "I am 'Antactican'. FALSE".

ja'a nai

"je'u nai" will mark something as untrue.

Then there's the metalinguistic negator "na'i", but this is used to deny the presuppositions of a statement. For example:

xu do za'o ralte lo mi cukta
Do you still have my book?

na'i go'i .i mi no roi ralte cy
No, I never had it.

If you just say "na go'i", that may be true, "I don't still have it", but you would be accepting the implicit presupposition that you did have it at some point.

That would be good for responding to "Do you still beat your wife?"