There were no serious economic sanctions for Crimea IIRC. Only toothless blocking entrance to the US and EU for some individuals.So depends on how much you enjoy sanctions. Can't really force Russia out but Russia can't force people to trade with it either.
They say don't accept the countries with unsolved territorial disputes into the alliance.The main problem really is say some future Ukrainian president convinced NATO to let Ukraine join.
There's Russian troops on Ukraine's territory (Donbass, Crimea) and I'm sure you know about what the articles on NATO say about that.
Not to mention, ongoing civil wars.