1) Signup for spamcop and route all your blackberry email through them, or if you are handy try setting up your own email server with Spam Assassin, that is what I did and I never see any spam on my Berry.

2) If you are with T-mobile phone them and request a 50Mb mail box, that is free of charge but might require quite a bit of calling around...

Also, try the filters at the web client, before I went with Spam Assassin I had over 50 spam filters.

The web client also has an "auto-aging" feature that will delete email after a certain amount of time, if you always read your email in a certain amount of time then this will help a lot.