That is a great idea from Franco about trying boiling water. It won't harm the aluminum, and just might save the grips for something else.
I used a thin, carbide tipped, small toothed blade on a cut-off saw to remove the top portion of the plastic grips above the aluminum tube. Started well above where the tube might be, and then cut off disc-shaped pieces until I got down to just above the metal end of the tube.
Then, as Matt suggested, I used a utility knife with a fresh blade to split the remains of the grip left on the tube. The pole was placed horizontally in a bench vise, with lots of protection by wrapping it in a tough polyester mesh that was too heavy for pack backpanels. Then the utility knife was placed on the pole with the unsharpened edge of the blade parallel to and flat on the pole, then pushed along the pole and against the grip to split the grip lengthwise. The idea was to keep the sharp edge of the blade from coming into contact with the metal and scoring it.
Once the grip was split, it could be peeled off the pole, often revealing a thick dried adhesive that looks like solidified gel. Sometimes this had gotten down inside the tube end when the grip was originally installed, and was difficult to remove completely.
This worked well with a number of long ski touring poles without damaging any of the tubes. When the weather warms up, they will be bent to make small camp chairs, but that is another story.
My biggest concern was not damaging a tube, but injuring myself by letting the utility knife slip while splitting the grip, or getting hit by shards flying back from the cut-off saw. Mesh lined protective gloves and one of those full face pull down polycarbonate masks would probably make sense.