Well, if you where running more current code (btw, anything earlier than 7.5.8 is susceptible to security issues) I'd tell you to setup Stop Forum Spam in control panel; the best you can do prior to that is install the modification at ubbdev.

As for if an email does off on a decline, I'm not really sure; you could try making a dummy account and deny it and check first hand.

Also, a CDN (such as CloudFlare) also helps block spammers.

I am a Web Development Contractor, I do not work for UBBCentral. I have provided free User to User Support since the beginning of these support forums.
Do you need Forum Install or Upgrade Services?
Forums: A Gardeners Forum, Scouters World
UBB.threads: UBBWiki, UBB Styles, UBB.Sitemaps
Longtime Supporter & Resident Post-A-Holic
VNC Web Services: Code Modifications, Upgrades, Styling, Coding Services, Disaster Recovery, and more!