Lately I've started to get emails that end in aol.com.

Is that a problem, and if so, why?

I'd like to know why ...

Since I have no idea what kind of messages you get, I don't know why you get them in the first place. There is nothing which prevents anyone from sending you an email, regardless from which email address.

... and how it can be stopped.

I cannot be stopped. But you could create a filter to delete them, or train your spam filter to deal with it, if that is why you're asking here.

Yes it's a problem. My email address does not end in aol.com. I don't have an aol email address. So I should not be getting emails addressed to me at aol.com. That is spam.

You say it is spam, but anyone can send mail to a BCC address. They can also fill in the to address. Often folks actually include themselves as To: if it is a club or whatever mailing list so everyone knows they snt it even though it is addressed to not them. Otherwise a BCC only email would have something like undisclosed recipients shown instead of a name in To:

Seriously I would suggest you just mark them as spam for a couple of weeks and the spam filter will work it out you consider them spam.