greasemonkey: an idea for the script

We all know greasemonkey, FireFox extension that can add DHTML to a webpage; so I was thinking if it would be possible to control/access cookies as well. Polish mobile provider Plus GSM has a SMS gate that you can use to send SMS for free; recently they limited it to only one (or so) SMS per day. The site leaves a cookie on your machine to find out when did you send the SMS last time. Removing the cookie by hand does the trick. But of course I'm lazy so I'd like to be done automatically.