There will be no difference perceived between disrespecting a person's ideas, i.e. not taking them seriously, and them feeling disrespected. My suggestion was to not take them seriously (the person who knows nothing but is being combative). To treat them as of they're raising a point that does not deserve respect or really much engagement, and that it's important to project this so that (1) they are not emboldened and (2) your actual audience, which is everyone else, understands that it's not a serious idea to be considered.

Also, you are much less likely to learn from a person behaving this way than you are to find that you must suddenly deal with disruption and bullying because you didn't clearly set boundaries and quash the ridiculous behaviors. It's great to learn from people, but you should prioritize doing so in a way that is productive.

I see a lot of folks on the left zero in on disruptive or reactionary people, thinking they'll turn them around or that they need to give them some special attention for the sake of "resolving" contention in a given space. This ends up wasting their time and often everyone's time while the contention is prolonged and the problematic individual fails to improve (after all, they're being taken seriously). This will put a strain on others involved as well, as rather than alienating the problematic individual you are now creating more situations for everyone else to become alienated. It's also just plain the least efficient way to learn and adapt and should only be done for strategic reasons, not as a rule.

Also... there are many people that have nothing at all to teach us except that we should isolate and avoid them. You are not going to find a valuable learning moment from the cryptofascist harassing every brown woman in the office. People that choose to spend their time trying to respect the cryptofascist to empathize with them and create a better project are going to be counterproductive, full stop. Make the space workable for the brown women and police the cryptofascist's behaviors.