[Sid Squid]

Either the threads are goosed from overtightening, or the captive nut into which the screw threads has come adrift, (it's got teeny welds that hold it to the strap - although perhaps not any more), and is spinning with the screw. Either situation will require their replacement. The clamps are nothing special, any clamp you can find that fits will do, but note the the groove in the rubber in which they sit is somewhat narrower than a typical hose clamp.
Carefully but firmly wiggle the carbs side to side while pulling upwards and they will eventually be wrested free of the clamps. Heat helps by softening the rubber, this weather makes things firm up a bit, so either run the motor 'til it's warm or perhaps use a heat gun - low setting obviously.