Here's a little trick you might want to try. Edit your registration form and add a simple math question such as 5 + 5 = ?, and make it a requirement that all new members must answer the question correctly.

It isn't exactly fool-proof. But then again spam bots are not fools, they're programs, thus they don't know how to proceed in this situation and can't get past the registration page. At least for the moment anyway.

I can't tell you how to do this on your particular board, but you might want to look for a forum modification to do this.

Does your board have ban filters? There are several boards available (some are even free) that have options to ban by usernames, email addresses, and IP addresses.

Ban the @zsdfg.com domain?

How you add to the ban filter depends on what software you use. There are 100's of boards available and they could all be different. So you just have to open up the ACP and see what is in the ban filter section. There most likely won't be any instructions, or at least not detailed ones anyway, but there may be something that will get you started. 

And by the way, if you ban an account then delete it, you open the door for that same account information to be used again. So it's best to not delete accounts. Just ban them and let then slip away into the nether regions of your forum database.

Yer welcome.