Had one (herd bull). Due to the circumstances it was difficult to catch him in the act. Tip off was the unexplainably poor condition of a number of the calves. Was very clever. Waited until the calf was nursing and snuck up to latch onto a hind tit. Immediately shipped him upon discovery. Suggest you do the same unless you can tolerate stunted calves.
Put the can't suck ring in his nose and see if that will curtail it. It won't break your bank to do it. If it doesn't work, do as holm25 says and get the cows bred and ship his butt. Is it only one particular cow or any of them?
These nose flaps cost about $3. If I see a replacement sucking they get one of these for life. Never saw one sucking afterwards.. saw them attempt it and get kicked in the face for their efforts (very satisfying).
BTW she calved last night. Seems to be a good mother so far.