How do I get my adult dog to stop biting?

My cocker spaniel is now close to five years old. She’s a great dog and completely trained except for one thing. Whenever she gets hold of something, be it a sock, or food she isn’t supposed to have she won’t give it back. And if you try and take something from her she will sometimes get quite nasty, and bite often drawing blood. I love my dog but I can’t continually have to be scared of her approaching her when trying to take something from her, what can I do to solve this problem?