Data caps are not speed limits in any way whatsoever, (so you could have a ridiculously fast service with a data cap of 10GB, you would still be able to download almost instantly, just not very much, like being able to order a couple of rocket propelled truck deliveries at most), they might enforce a data cap by limiting your speed if you go over, making it hard for you to download a lot of data, they could shut you down for the rest of the day if you download more than an imposed daily data cap, they could also shut down your account for the rest of the month.
As far as I know Comcast warned you first, and if that doesn't work, they suspend or cancel your account, and you would have to call them to get it up again.
Currently they are testing simply charging you for the extra data in some places, data cap has been suspended for now.