They are more than likely using some type of program such as xRumer or similar...
Check your logs, also check your session table, see if there are similar ip's trying to register at the same time.
Once you find out the ip or several ip addresses simply ban them via htaccess, you may also want to contact your host about this to see if they have any server level alternatives depending on your hosting account type.
I've seen this before, using some methods outlined here I was able to reduce this then the clients host helped as well -
https://vborg.vbsupport.ru/showthread.php?t=276547 scroll down to find "Session Table Issues?" and see if that helps.