The providers don't help you at all. Spammers run dictionary attacks on domain names to come up with new user names.... So you are on your own sadly:
There are several options.
1) You can pay for a Spamcop.net account, forward all your email to them, and then to the Blackberry.
2) You can configure your own email server (that is what I did), make sure it has decent spam protection (through for example Spamassassin), or you can purchase service off a hosting provider (like www.guesshosting.com
the people that host this site). They offer email with spam assassin and you can get an account for just $30 A YEAR!
3) You can setup 100's of filters in the webclient, this would need you to add and alter filters on a daily basis.
But most importantly, never ever give out your Blackberry email address. You are better off getting a free Gmail account and forwarding THAT to your Blackberry account.
The most ideal situation is a mix of 2 and 3. Get your own domain name (you are looking at around $7 a year here) and then host that with above hoster (for example) , then forward all your email (post spam check) to the Blackberry.
Through Spamassassin I get fewer than 2 spam emails PER WEEK in my inbox. I still check the spam folder every now and then for false positives and if needed add them to my "safe" list.