...I can fire him. I mean, obviously I need to go through the proper HR procedures, follow local employment law, respect the terms of his contract, etc., but I have no obligation to keep him employed if I think he's a net negative to my company. Arguably I have an obligation to not keep him employed. And I certainly have no requirement to wait until he has been convicted of a crime; it's not even clear any of the above is a crime.
Similarly, if I run a resort, and one of the guests is so unpleasant they're driving away other guests, or if I run a restaurant, and one of the guests is making other guests uncomfortable, or I run an apartment building, and one of the tenants is behaving poorly.
A university is a business. People come and pay money in exchange for a service, and if someone is interfering with the ability of other people to receive that service, or making people reluctant to come and pay money, it's perfectly fine for them to be asked to leave. If what they're doing happens to be a crime, then by all means, also report them to the police, but it's kind of odd to suggest that you shouldn't expel students except if they've been convicted of a crime.
That being said, I think universities are doing a terrible job here, and I think that given the extreme importance of a university education in modern life, the disruption that being expelled entails, and the stigma that will follow someone if they are expelled, we need to be very careful about the process here. My local dollar store will ban you from the store if they think you shoplifted, and I assume they get it wrong sometimes, but it's fine, because being unjustly banned from a dollar store does not ruin your life. Being unjustly expelled from university could! Things are broken and need fixing. But a world where you can only be expelled on conviction of a crime makes no sense either. The discussion need to be about the correct amount of protection needed.