I have a tenant who is harassing another.  

And they'll probably have to increase them even more.Then there was the plant nazi who harassed me about my tomato plants planted in pretty ceramic planters....OMG, I couldn't get out of there fast enough!

I would give them a warning that "house rules" do not permit disturbing/harassing other tenants and that there have been some complaints.  

They change the rules (including whether or not you can continue to rent out your unit), they change the amount of the dues, they harass you about the plants you have on your patio....Take it from one who bought a condo expecting to live there forever, and sold it two years later - because of the HOA.Condos are also harder to finance, so they're harder to sell, too.

Tenant A routinely complains about tenant B making noise, blocking the driveway, and having multiple people over, and when I recently brought it up with tenant B and asked them to keep it down and generally be more neighborly, as stated in the lease, they complained about tenant A harassing them and acting aggressively.  

That's giving her a license to harass the other tenant, in my opinion. 

In which case you would have not only claim to property damages but also several avenues of personal damage (i.e illegal seach, false charges, false arrest, possibly harassment and/or defamation of character.)It's no more the tenants fault then if the property was vandalized or broken into, they have no control over the police, and they had done nothing to bring the police to the property.

You don't want to inspect so often that they can say you're harassing them or affecting their quiet enjoyment, but you might be able to detect something going on.Funny story: We knew we had a tenant with a rabbit (no pets policy), because the owner hauled trash away on weekends because we didn't have city pickup on weekends. 

He has allegedly been harassing her and threatening to move back in because he is on the lease.

The CAUSE of the issue is immaterial - it's the noise and harassment issues that are lease violation.