This addon does check the about field for keywords and URL's:
From (simple) multiple accounts detection to isp/connection fingerprinting with score-based moderating/rejecting logic. These are very effective low-hanging fruit at reducing spam. Supports migration configuration from the following XF1 add-ons...
xenforo.com
I cant find a way to check existing accounts. As its a common spammer tactic to later add the link to the field I wonder if there is some way to check existing accounts.
@Xon is this something that is available or planned?