I've heard cl_allowdownload 0 only turns off custom sprays for some reason. There's several threads b|tching about the sounds over at steampowered forums, with many admins some how claiming its their god given right to require me to download and listen to beavis and butthead sounds while playing on their server.
Bad news: There appears to be no way to stop them from downloading. Everyone is requesting the option from Valve for the next update, who the hell knows if it will happen.
Good news: Once they download once, you can open the files and delete all the data within and save it as an empty file. It won't download again and it won't play either. This sucks since its kind of a pain in the ass to do, especially since every server admin likes to save his junk files in a different location to be 'unique'. There's supposedly a 'anti sound pack' that does most of this work for you out there, but its just the same thing as what I suggested.
I feel your pain, and hopefully valve will do something to fix it soon.