huge activity log files

Hi,

We get lot of spamming , junk emails. WHMCS makes a lot of huge log files. It is not easy every time delete these log files from data base.

How can I delete activity log files of selectable users , emails ? I want to keep log files only for clients related. Others want to delete timely like 15-30 days.

I have to plan complete emailing system via whmcs.
Please guide me.