Dribbley Outside Taps.

My house has 3 outside taps. Each of these taps has a green hexagonal thing on it that dribbles water every time I use the tap (not too much of a nuisance when I am filling a bucket but irritating when I want to use the hose). I can turn these hexagonal things by hand and they look like a large nut but they don't unscrew when turned these just go on twissling. I was told by a friend last weekend that these things are some kind of frost protection device.
Is there anything I can do with these things to stop them dribbling short of replacing the whole tap. I don't want to continue to dampen the ground around the taps every time I try to use the hose.
Just to be clear - The tap itself doesn't drip when it's turned off, it's these hexagonal attachments that drip when the tap is on.
Thanks in advance for any help.
Regards Mark
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Those are anti-siphon devices. They keep the plumping in your house from sucking in hose water if you should lose pressure. Depending on who makes the faucet there may be rebuild kits for them. If you squeeze the anti-siphon device hard enough it will probably unscrew. I have know people to unscrew them, fill them with silicone, and the screw them back on (not that I'm advocating that...).
Mark Sladden wrote:

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