Call in an authority on it. If you get the cat spayed without their consent not only could all hell break loose, but they will just get another one, and so on.

Call the CPL in to help or the "dare I say it" RSPCA!!!

It is hard to see happening but in all honesty there's not that much you can do, but the charities may speak to them and hold more weight than you do. Trust me though, if you remove that cat they will get more, I have been there and done that, and am now gonna resort to the advice I gave you.

Other than just keep saying "you are being cruel " to them, I dunno what else to suggest, BTW who the F*CK pays 50 quid for a moggy kitten? That's ridiculous! Find out if it is a pet shop and have a go at them, maybe. Unless it's dangerous to.