As far as I know, WMP9 has no way of selecting or deselecting file associations.
There really is no need to *deselect* from a program, you simply *select* in the program of your choice and the other application is automatically deselected. If the application does not allow you to select file associations, it's possible that it can't handle all files of that type. But many programs I think are conservative or just incompletely specified w.r.t. their file associations. For such programs, use the previously-mentioned applications (Resco Explorer or GSFinder+) to specify the file associations you prefer.
There is a list of audio/video programs at
http://www.feederreader.com/mediachart.html that may come in handy. On the chart is an attempt at identifying programs that can handle streaming based on information gleaned from various web sites, although information on streaming is difficult to round up. If you find out more information about streaming and specific programs, let me know!
Good luck!
Greg Smith
Author, FeederReader - Pocket PC *direct* RSS text, audio, video, podcasts
www.FeederReader.com - Download on the Road