Aside from, "Just don't do it", I can offer the following scary suggestion:

Be transparent.

In a real, open, honest relationship, you can tell your girlfriend that you received these photos, but told the girls you are taken and to stop. She might freak out a little, but it will show:

- That you're open and honest, and you TRUST HER. This will lead her to trust you more.

- That this is how you approach a relationship. No secrets. She will, in turn, feel she can do the same with you. If a guy ever comes onto her, she can tell you it happened, and that she told him she was taken and to stop, too. And you will believe her.

- It will remove the temptation on your side. You won't be able to act on it because your GF knows, and secondly, it makes the whole situation less of a hidden, exciting, on-the-side deal. It'll just be what it is.

Now all of this depends on one thing from you first: You need to tell these girls thanks, but no thanks, you're with someone. Block them if you have to. After that, you can do the above and be a man of your word.