You could install:
Firewall: CSF, APF, ASL
Protection Tools: MOD Security, MOD Evasive, fail2ban, BFD
Importent: Keep your Software (Plesk, PHP, WordPress …) uptodate. Running PHP in safe mode (could help maybe)!
The best thing is to buy Atomic Secured Linux
It is more than a Firewall! Its a full package of security and always uptodate! I love it!
Just install ASL and you are done