You can not do this. You can either change it to a new address or leave it as it is. The only other way is to uninstall and reinstall subsonic on the android. I had the same problem.
just overwrite it with a blank one, you do need to specify a url in correct form so just use something like "http://add.new.url.com" or something, that will wipe the old url out and place that on in which you obviously won't be able to connect to.