I was getting bombed by lots of ridiculous sounding new members a couple of months ago. It stopped dead as soon as I removed the existing Question and Answer Challenge entries and put some new ones in. Make them something specific to your site theme - something that your potential members are likely to know.
At the simplest level, it would be a nice feature if community members could have the ability (if admins allowed) to have their profiles viewable by guests or not. It started with a few members complaining that the world could see their profile pictures or other stuff that they'd put on, then a few more chipped in to say that they'd customize theirs a bit more if only members had access. As a result of that we ended up turning off profile view for guests - where other members didn't mind, or actually wanted their profiles visible to everyone.
I suppose that making it a bit more complex like 'only allow friends' or choosing which bits to show would be a bigger ask, but stuff like that might be a bit more 'community orientated'.