They may just be well and truely stuck! I don't know of anything else that holds the bottom half on, but check for a screw or something inside.
If you are sure it is only the rubber ring, rock back and forth and twist slightly until it comes loose. Don't pull hard enough to bend it!
Check for an emission control hose on something down underneath, too -- I don't know if there is, but a stiff hose could hold it on pretty tight. I know the blow-by hose is on top, at least it is on the KE-jetronic fuel system.
Mine was floating around with two of the three mounts separated, so I didn't have any trouble at all removing it!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!