Removing thousands of spam contacts

[btc97]

Is there a way of searching for spammy registrants so they can be deleted in one go? I have thousands polluting my database after a registration form has sat unprotected for a few years.

Also, is there any alternative to Captcha or Recaptcha for protecting a contribution form? I think our users will struggle to complete them.

[Pamela]

Is there a common variable (or variables) between these spam contacts?

[btc97]

There are some similarities (e.g. only a few fields filled out), but there is overlap with non-spam contacts. I was wondering if there was a way to run through contacts to look for suspected spam users (I think WordPress has something like this for existing spam comments for example). I think I would have heard about this by now if it existed though!