Odd error.log content on 1 server

I have disable_functions line but exec was removed due to issues with Centovacast. I could try adding exec again.
Indeed, remove execution rights is also a possibility, stupid that I did not think of it.

Thanks for the tips.
I also think the user might not aware of it being running. I do have maldetect running, munin and CSF/LFD so if something really goes wrong I'll be notified. But it would be better to find out which user is causing this.
Probably indeed by disabling it for the users, somebody will complaint or I might see something in the logs.
Thanks!