No bit rate does suggest fakes. I normally filter out wm files myself. And most files under 2 MB I give the junk tag to.
To filter out certain extensions ... go to Tools > Options > Filters > Keywords. For example, add .wm
Oh and on the idea of filtering, see some pointers here How to find music & also Direct instructions to eliminate porn from results

(There are some gnutella clients who share audio files that for some reason sometimes don't show bit rate .... I can't remember which clients .. shareaza or morpheus, etc.)

It might be worth adding this to your LW: Hostiles List so it helps filter out known spam ip's. This list needs to be kept up to date btw so renew it occasionally. See if it helps or not.