Hello!
Here are two ways to do this:
1. Tick "At the start of the program clear log records older than X days", but this will clear the logs after you start MF Pro next time.
2. If MF Pro is not running at the moment, then you can manually delete the folder with all the logs. They are stored here:
c:\Users\%username%\MarketFeeder Pro 8\profiles\%profile name%\logs\
You can choose to delete all the logs, or just trigger logs or only program logs. In that case, choose the relevant subfolder, e.g. "logs\triggers".