Spam User accounts can sometimes be added to a site through a WordPress registration option. This may be evident by User accounts that appear in the Members section of WishList Member, but do not have a Level assigned to them.
A WordPress registration option can be disabled to prevent these types of sign ups. You are able to turn this WordPress option off while still allowing members to register through WishList Member as these are separate options.
Unchecking the “Anyone can register” option in the Settings > General section of WordPress can reduce these types of spam registrations.