E-collar is an "electronic collar" or you could call it a shock collar. There are a couple ways to break your dogs from chasing deer. One way is to drive around until you see a deer run into the woods. Take a dog out of the box and put him near where you saw the deer. If the dog starts to sniff the trail from the deer, correct him with the collar and then let him sniff a rabbit fur or rag with rabbit scent on it.
Another method is to use breaking scents. You can buy (for pretty cheap) small felt tabs that go on the dogs collar. Saturate the tab with deer breaking scent. After a while, the dog will want nothing to do with the scent and will shy away from deer scent. As a last ditch effort (you have tried these methods and dog still runs deer), you can take a small amount of the deer breaking scent and put it in a small syringe. Squirt a tiny bit in the dogs nose. It will be awful for the dog, but it is a last ditch effort to protect the dog. In many places it is illegal to run deer with dogs. Also, deer run differently than rabbits. If your dog is chasing deer, it can easily run out of your range or even run to a highway, etc. If the dog still runs deer after all of that, it is time to find him a new home.
OMG that is awful!! How about just containing the dog safely so it can't chase deer in the first place?
That is also my recommendation for the OP. Fence your yard so your beagle can be outside without being able to roam. If you can't do that, build a run instead. Containment is the best, most effective, safest and most humane method for stopping your dog chasing rabbits.
A dog with a high prey drive like that also does best if he is getting drive satisfaction, does he like to tug?
my 8 year old beagle got onto that trick pretty quick. If she's not ready to come in she'll come up close enough to see i've got the rabbit and then take off on another track. With her the only way to leash her up is to speak VERY sharply and make her go to ground. Once she's on a hot track she won't stop for anything - unless I get close enough to yell her down.