Use the unlink tool, check the help file

.
Or real quick, look thru the tool bar, find the one that looks kinda like to donuts linked together. Select it, click on the video, click on the audio, it'll unlink them, then delete the audio track...
It'd be a good idea to not have any audio. I would use virtualdub to wait, I just exaplained this the other day. Audop|No Audo, Video|Direct Stream Copy, File| Save As Avi.. whoosh done.
~klinky