Block email address / domain from using email form

[Adrian28uk]

I keep getting the same person spamming people on my website through my email form.

The email address is they keep using is [e-mail] but this sometimes changes to a different name but still from the o2.pl domain

How can I block anyone using my email from the domain name o2.pl

Many Thanks
Adrian

[etully]

if (ereg("@o2.pl",$email)) { exit; }