So this other realtor (or anyone else for that matter) can't take the photos and use them for commercial purposes without the photographers permission, corrent?
Two questions here, and probably more.
No he can't steal your photographs off the web and use them, especially if they are off your website, because the minute you post them, on your site, they are copyrighted. If you paid for the photos they may be yours, or they may still be the photographers, depends on your contract with the Photog., but that isn't the major issue here.
If he didn't get the photos from you or the photographer, NO he can't use them, in any way.
If he bought the right to use them, from the photographer (which from your original question, I'm assuming he did not) then he could use them.
Some people just don't understand the law at all, and think, if it's on the web it's free. It's not. It's that simple.
First step is notification. Not threats, just notify him that he is illegally using your images, and he needs to remove them. (a cease and desist request) If he doesn't respond and doesn't remove the photos, he's in violation of the copyright laws. Here's where it gets a little more complicated.
First you have to figure out who actually owns the photos. You or the photographer. Unless the photographer sold them to you, you are basically just paying for a use license.
Then, should you want to sue the guy who's using them, you'll need to formally copyright the photos. Even though your website is copyrighted and the laws say that the photographer has rights, if you need to go to court for damages, you need to register the photos BEFORE you file a claim.
Which translated means the photographer has to register his work, and the photographer is the one bringing the lawsuit, not you, because the photographer probably owns the photos.
Hope that helped?
1) Ask nice, and explain they aren't free and you have paid for them and have the only license to use them.
2) If it's worth your time, find a lawyer who will write a letter, on his stationary, warning the other agent, that if he doesn't stop using your photos, you will take legal action. Depending on the lawyer or if you know one as a friend, a letter might cost $50 or nothing.
3) The lawyer will tell you if it's worth taking further action. As in, you can't get blood from a turnip. If it costs $10,000 to win the case and you get nothing, it's not a worthwhile action.
4) Next the lawyer will tell you, that before you can file criminal charges, which wouldn't be you, but the DA or the local authorities, or file for civil damages, you have to legally register the photos.
Do #1, it usually ends right there, especially if the offender made a mistake and doesn't understand the laws or free use. If he's a jerk, #2 will usually show you are serious.
Alternate plan. Go get cousin Guido to knock on his door, and explain that he won't walk so good with a broken kneecap.

(just kidding!)
You can also have the photographer contact Craig's list with a complaint, and contact the website that's hosting the photos, if they aren't being stored on Craig's list. That will often cause them to disappear.
Here's one more possibility. Is he linking to your photos on your site? That's theft pf bandwidth. A nice entertaining trick is change your site to renamed copies of the photos, and change the original photos to something that says "(insert agents name here) steals photos." with a photo of a pig or a horses back side.