Windows live mail – Delete old emails and keep them on my server

Here is my problem. 

I have almost 25000 emails in my gmail account that i sync with my WLM. So I ended up with this massive email folder on my computer that is over 8gb and slows down any scans (virus, searches etc.)

I just want to keep my WLM current, like the last week.

How can I delete all my old  emails from my WLM and still keep them in my gmail? Everytime I delete something it automatically syncs the delete on my gmail. And of course, when i do this I do not want it to be re synced. 

Any advice? 

Thanks in advanced!