plugin for banning an IP address?

You can put the IP in your comment blacklist or try this: https://www.clockwatchers.com/htaccess_block.html

but how can i find out what their ip address is? is there something that will identify that particular users ip address?

Depends. How are they abusing it? If it’s via comments, go to the comment section of the admin side (where it lists ALL comments) and you’ll see the IP address below.