In a contract between businesses, implied terms can be explicitly excluded, but not unreasonably. (Of course, the seller must still have the right to sell the goods.)
If you are selling goods to a private consumer, you cannot alter their statutory rights. Attempting to do so - for example, by putting up a sign in your shop saying 'Goods cannot be returned' - does not prevent consumers from exercising their rights and may also be illegal. You could be prosecuted.
