Welcome to my old frustration... all the unhelpful suggestions about "Just pry up the foot!" after you've spend more than an hour trying to do literally that. In my opinion, BTA's are just a more attractive, less invasive version of aiptasia, and I will never have one in my tank again unless it's dedicated to nothing but BTA.
To the OP:
There is no easy way to do it for any nems that are anchored deep within the rockwork. There are some non-easy things that you can do, but fair warning, they are time-consuming.
First, a few tools you will need: hydrogen peroxide (12% if you can find it); dental picks; old credit cards; nitrile gloves. And a LOT of time.
If you're at your absolute wit's end with them, hydrogen peroxide will absolutely irritate them and may (may!) convince them to leave their anchor point. This is best done out of water, but if you can't get it out of water it can work with all flow turned off. Get a dosing syringe with a luer lock tip, a nice thick gauge with a long needle. Fill it up (about 3ml worth) and apply around the edges of where the 'nem is attached. Don't inject directly into the 'nem unless you're trying to kill it. And for the sake of the rest of your livestock, you don't want to kill it if you can avoid it.
To make it easier: Use a dental pick to pry up a section of the foot, and squirt the peroxide underneath. This causes bubbling that they find unpleasant (understandable, since it's also lethal) and may cause them to detach. Keep working with the pick to dislodge the foot. If you can get a pick with a somewhat chunkier "bar" instead of the finer hook, you can also use that directly. 'Nems don't like the feel of steel, so if you can get the steel under the foot and work it around, you can again sometimes dislodge them.
The credit card is more to hold the dislodged foot in place than anything else. If you're this far gone, you probably don't have access to slide the card underneath the foot. Constant wide pressure - again, you're trying to make the 'nem unhappy where it is and like it would prefer to be somewhere else.
And the gloves are because... well, sometimes you just don't have an alternative but to get in there directly with your hands. 'Nems are pretty slimy and can sometimes slip off of your bare skin, so the gloves help you grip it better if you can actually manage to dislodge it.
If you go this route (ESPECIALLY if you're using peroxide), don't try to work on more than one 'nem at a time. Peroxide can be dangerous to the rest of your livestock if you use too much of it, especially if you're using 12%. That's one of the reasons I advise using it out of water or during a water change. It will naturally dissipate back into regular water over time, but in the time after your initial application it can be dangerous.
