apache server under attack

Hi all,
I run a small community . Iam using apache web server with smf forums on a vps with centos 7. I have all good firewalls going (csf) . Iam trying to solve an issue with a user crashing my7 forums. The memory on the vps and cpu will spike to 100% and create a "cannot connect to mysql database" error. on the apache error logs I get a mpm_prefork error showing AH00159 error out of memory. can anyone tell me how to prevent this attack. thanks.

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