To reduce spam, you need to use those spam mitigation controls in the ACP. (Your username is blue, so I assume you're admin.)So
If they come through, well, two things: IP Ban/Discourage along with discourage/ban on their accounts. This way, even if they slide through the spam mitigation cracks, you can take away their access. The more you do it, the easier it becomes.
Just an F.Y.I.