Step 1:
Turn off Free Mode by going to Admin -> Tools -> Membership Levels
Step 2: Turn off Promotional Membership
Step 3: Click on "Non-Members" It's in green below
Step 4: Disable all actions for "Non-Members"
Step 5: Disable all actions for "Promotional"
Step 6: Enable all actions for "Standard"
Step 7: Upgrade your sites e-mail system
Step 8: Install an anti-spam filtering system
Step 9: Install a Language Filter to deal with anything (like url's) that slip past the anti-spam filter.
Step 10: Go to Settings -> Membership Settings Remove checkmarks for Automatic Profile Activation & Automatic Profile Activation without Confirmation E-Mail then click Save at the bottom of the Table
Step 11: Install an IP Tracking system to catch multiple accounts from the same IP
This will do a good job of dealing with spammers/scammers and clean up your site for you in the long run.