I am getting spam that normally gmail caught on my main inbox, is there a way I can create a filter rule to get the spammy email to trash?

So basically what I want to tell kmail is to get all the email belonging to Spam/ away from showing it on the main box.

Regards?

You could if you knew if GMail flagged the mails somehow. Get one of these messages, hit "V" to display all headers, and see if GMail adds some that are spam-related. If there are some, you can use those to create a filter to get rid of them.