Here in aus they are either bolted, or riveted. if they are bolted you can undo them, but you'd have to break the rivet otherwise. Some new ones that get taken frequently (eg valentine st) are attached by some diff method. I guess for those you just have to hacksaw it off or something.
If it's worth doing, it's worth overdoing.
cheech on totse2: http://totse2.com/member.php?16551-cheech
A friend of mine once found a sign post that was held to the pole using a steel strap. As my friend had a set of bolt cutters in his van it took him 2 seconds to collect the aluminium sign which had a scrap value of around £3. Not bad for 2 seconds work my friend thought...
Three kids who were not me and my two friends drove to a small shitty town and used sockets and crescent wrenches and just took them off in clear daylight. Just remember, those fuckers are bigger than they look. Make sure you have a sheet to cover them while in the vehicle.