I remove some infested limbs (usually smaller) and shave off the larger leaders or limbs to keep structure. If I shave it off with the saw, I am cutting down into cambium, then apply a spray pruning sealer over the wound. This will help kill any of the mistletoe roots that might still be there. EIther through light deprivation or the mere fact of it being a petroleum product
If the infestation is too bad, you usually have to amputate ....
I always try and find something to save in a top or limb cutting back behind the infestation to the next viable node.
The shaving of the infected area and coating it with the sealer (tar) has been very successful for me.
I have trees treated this way over the years that have become vigorous again with nary a sign of the parasite. I had a couple trunks try to sprout that I had not cut in so deep for aesthetic reasons (eye level across from house). But the sprouting was minimal and the treatment was applied again after boring each sprout out with a knife tip. Or spooning it out if you can picture it.
There will always be some latent seeds that will sprout on tips and often at the branch collar. The birds plant it anyway. So seeding can come from afar via bird droppings.
I set up the customer to a 2-3 year cycle of maintenance on the tree. You'll catch any new infestations or latent seed sprouts in that cycle. Any restructuring of the tree can be addressed as well.