They can't expel a member, but if playing strategy video games has taught me anything, most of the members already have a secret text group or chat channel and are coordinating the creation of a new alliance to be announced as soon as they all jointly announce their withdrawal from NATO.
Maybe sans USA, Turkey and Hungary. But again I am basing this purely on my own direct experience playing co-op strategy games which, while technically set in space amid warring alien races, still feels about as apt a qualification as any actual world leaders are likely to have.