This is too vague to really offer any meaningful advice about, but:
even if they have good praxis or ideologically align with our goals: they're advocating for ideas which would undermine our work in the longterm.
Say this person wasn't in the picture, are you on track to building socialism/communism/a better world in X number of years? If the answer is yes, great, carry on, you're doing brilliantly. If the answer is no, perhaps it's worth considering where time and effort might be better spent than chasing away someone who you've decided has a slight chance to undermine the goals that you aren't actually in any position to achieve.
Sorry if it's blunt, but this mentality is why I struggle to hold onto hope for the western left. Everything reeks of online brainworms but the vast majority of online discourse is completely irrelevant to the outside world. We only try to embrace the most terminally online thinking among us, despite those people being the least capable of actually making any lasting change in the world. We pretend we're building the future and this person with the wrong views on outdoor cats is going to spoil it all if we don't drive them out, but we aren't building anything and we just lost a potential comrade.
I really am starting to believe he's stupid enough to do it. And that this timeline is fucked up enough that he might well get away with it.