Yes, they are threaded. It was very easy for them to get crud built up in them then stuck in the on of off position.
I've soaked them in a carbon cutter/ cleaner to get them to move enough to turn on or off. Once they start to move they usually clean up quick.
I've not removed one, but the best advice I've read was to insert a nail through the holes then turn it off with a claw hammer.
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