The scammers usually take the money via your cellphone bill, it's devious, immoral and underhand, but apparently not illegal!
A friend of mine was caught out this way, and it cost her £9-50, but fortunately she stopped it before a second payment was taken.
Never, ever, ever give out any personal details, I know we're all aware of not giving out banking information, but you can enter a competition without ever realising it will cost you. If, when you unwittingly enter a 'scam' competition, you use your cellphone number, the charges will be claimed from your cellphone bill, and your provider will not prevent it.
You need to contact the 'scammers' which is never easy, and tell them to cancel any 'competiton entries' you have made, and remove all your details from their database.