Along the same lines as Mark's dog training:
Keep in mind that not all bubble tips are the same. I actually think there is enough diversity for some to be labeled a different species or at least a subspecies. As such, some are inherently more likely to move than others. The reddish footed RBTAs for example are movers whereas the white footed ones are not.
Try to get one from a reputable breeder whose nems typically don't move and you'll be in better shape. If you start with good stock its a lot easier to train em
. My nems will only move after a split as they tear apart from each other, they both wander about a bit looking for a good spot. I don't bug them while they're healing, but once they are healed I gently coax them to the spot I want them in with the following:
As such I use positive reinforcement, negative reinforcement, and physical barriers to keep my nems in the spot I want. The initial step is to place the nem in a spot where its foot can be wedged fairly securely in the rockwork. Mind you, most BTAs like to hang upside down at night, so an overhang is preferable.
If the BTA is on the spot you want it to be in, and extending the direction you want it to, then feed it. This is the positive reinforcement portion. If it comes out in a manner that displeases you... don't feed it. Of course if you have clowns that feed your nem, this doesn't always work too well
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With the negative reinforcement portion, if its somewhere I don't want it to be. I will use a stick and detach the nem's body from the rockwork forcing it to contract back to its foot. I then poke the foot a few times with the stick for good measure. Make sure you tell the nem its being a "bad nem". Try the Cesar Milan "shhh" as well.
As for physical barriers, most corals cannot stand up to a BTA. The one exception is a milli. I find that the BTA will actually retract its tentacle once it hits a milli. As such, I have a lil forest of millis surrounding my BTA area to keep it "fenced" in. Sure the BTA will sting the milli, but the milli will sting right back and there seems to be "detente" between them if you will. I don't have any hydnos, but I imagine they might do well against a BTA as well.
G'luck.