One trick you can try if the BB spindle is hollow is to get a long bolt with the right thread, and a washer use it to hold the tool in. It might take a bit of finagling, but isn't hard (the hardest part is sourcing the screw). With the tool securely held into the cup you should be able to torque it more without it camming out.
Otherwise, you can try fitting a block under the "hub" of the tool or wrench, so you can apply torque without the tool moving down.
Lastly, if you can improvise something, take a hint from people who have to remove tight lug nuts on cars and trucks and find a T handle or 4-way tool, so the torques are applied up and down and don't push the center across.
All the above is for extremely tight cups. In most cases I do fine simply holding the center against the sideways force with one hand, while torquing with the other.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.