Can an alum or bar member un-recommend me?

I got on the bad side of someone whose dad is an alum of a school I want to go to. He is also a member of the bar.

The person whose relations I have soured is also someone I had previously confided in.

Can the person or his dad talk to an admissions office or the bar and say bad (or false) things?

I don’t see any reason he couldn’t at least try to contact these people.
The real question is will the admissions office listen to what he has to say.
If you’ve done something clearly unscrupulous and significant in nature and he can prove it the bar may very well be interested in what he has to say.

What things are unscrupulous and significant?

Didn’t we just do this with an internship?

Of course they can call. Whether anyone gives a shit is the question–one that is entirely subjective.

Well the matter I had confided in the person is something that that person later distorted and disclosed to someone. It hasn’t done major damage yet.

I got on his bad side by making some litigious hints. If I actually bring a defamation suit, will that make him and his dad more or less inclined to call the admissions office/the bar?

That is, another question is whether I can do anything to prevent harm.