You can define custom rules for the SpamAssassin spam filter. However, we recommend you only make changes if you are familiar with the technology; the syntax used for SpamAssassin rules is difficult to understand otherwise.
Please note that you can invalidate the entire spam filter by adding even a single faulty rule.
Detailed instructions on SpamAssassin syntax can be found in this PDF file.
You will find advanced mode in the spam filter settings of My Panels.