I took my witches hat/cable exit thingy off yesterday. It was screwed down and glued with Sikaflex.
My solution, which worked well, was to use a stiff paint scraper and a rubber mallet. Start by bashing it into the joint at about 45 deg until it appears on the inside, and then follow the joint around.
The scraper has to be a stiff one (as always) not a flexible filling blade. Also a rubber mallet seemed to be better than a hammer.
I cut through about a foot and a half length in a couple of minutes, no swearing at all.
"A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools" Douglas Adams