I cannot find a way to "undo LGTM" if I was to quick to give approval. Working at a company where I have to LGTM commits literally every day, believe me that it is necessary every now and then.
it would definitely be trivial to add the ability to rescind an approval with a -1, for example. Unfortunately I'm a bit bogged down with my other OSS projects at this time, but I would be happy to accept a PR. In fact, this is the function you would want to update https://github.com/lgtmco/lgtm/blob/master/web/hook.go#L100