Yep, in protests/events, sometimes you meet people who hold views with which you disagree and some which you may even find personally offensive. I suppose there's a few options for how to manage that, and different people will have different approaches.
In an 'all under one banner' approach, it can be a bit easier to enforce if you have pre-agreed sign and speech content or themes - that way you can say to people that they need to stick to those pre-agreed things and avoid going 'off-message'. That works fairly well at smaller events where you might have sign-making sessions and other preparations being done as a group, for early intervention - which could range from a tactful chat through to a "well, that sign can f*ck right off".
In my experience of working with people who hold prejudiced or extreme or otherwise hate-filled views, those views often have their roots in a person's own experiences of life and are often not consistent or reasoned (because hatred isn't). If someone isn't posing an imminent risk of harm or serious offence to others, then engaging them in a discussion to explore why they feel that way can sometimes help lead people to question their own views. It might not change a person there and then, but sometimes you see people a few weeks or months later and they have changed their views. Sometimes, though, all you can do is tell someone you're disgusted by their views and don't want to hear or see it anywhere near you, and ideally you'd rather they went home and took their bigotry with them.
If someone is insistent that they're going to carry an offensive sign or otherwise cause a risk of disruption or harm, then the 'official' advice is that it's a police issue once they start causing a public order concern, so to report it to officers rather than getting into further confrontation with someone who is clearly looking to offend or provoke a reaction. However, that doesn't tackle the issue of the potential for distress and harm from their signs, speech and conduct prior to removal. So, what would happen with us would be messages sent out to the online groups being used to coordinate people attending, so that people would have the choice to attend or not - eg "The Fascists are under the bridge again" - and we'd think about how to keep them away from anyone potentially vulnerable prior to them being removed - eg arranging for certain groups to have assembly points further away from them. Not foolproof, I'm sure we've got it wrong many times, but we've tried and we've tried to take feedback on board from when we've got things wrong.