I just removed about 20 manjos from a Live rock only tank, that is a holding tank while I build and set up my bigger tank. I had a pair of old kitchen scissors that were old and kind of dull. I turned off the return pump, and reached in with the scissors and "pinched" the manjo's under head by their stalks.
After holding the scissors around their stalks for a few sections, they seem to "release" their foot, almost their first defensive mechanism is to retract, and when they can't retract because your holding their neck with the scissors, they let their foot hold go. I was actually amazed that they manjos didn't simply cut in half, but even with pulling hard the manjos didnt cut in half and came out as a whole. I removed over 20-30 manjos this way two nights ago, but the hard part is getting rid of the remaining 10-15 manjos because they are situated either deep in a crag or somewhere else where I can't get the scissors.