If they're only e-mailing the group (not approaching you directly), I'd just ignore & delete. If you're being individually targeted, send back a reply saying No Thanks - Not Interested. Lather, Rinse, Repeat.
If they start getting pushy, go to HR. A lot of companies have policies against any kind of solicitation at work.
If your work relationships can tolerate it (this can be tricky), you could send out an e-mail of your own requesting to be left off everybody's "potential customer" list.
And remember......No is a complete sentence. Good Luck!