OT, bandwidth questions

I need some help and since there are a lot of computer savvy people here, I'd like to ask a couple of questions.

Our website had been working fine until we added in a forum. We were immediately hit with spammers who did manage to infiltrate it and fill it with crap. We got that cleaned up and got it set up so a potential member has to get past me first, meaning I get an email saying this person wants to join. We have deleted all of them because you can tell by their user names they are spammers, lots of weird letters with two numbers at the end, all of them are like that. So, there are just a few members, who are legit.

We keep running out of bandwidth which of course means our site is blocked to everyone. Not good for sales. Our domain host has increased the bandwidth three times now and today we exceeded our limit again. The site is now up and running, and the domain guy is looking into what's going wrong that this keeps happening.

My web design person is going to take the forum down for now. We had no problems before we put it up, so I think starting there is a good thing. A friend has suggested our site has been hacked and is being used by someone for some nefarious purpose. Is this likely? If so, what do we do about it?

I am rather frustrated that it's taking too long for my liking to have the forum come down and to get some answers as to what the problem is. I've been googling but not getting anywhere, so I thought maybe someone here would have some ideas for me.

Thanks in advance,
Funkynassau