I don't know why you're getting downvoted, it's exactly right. Poland agreed to adopt the Euro as one of the conditions of being accepted, and then the global financial crash happened and basically said you know what guys, we won't be doing that after all. If a country wants to stop using Euro they absolutely can - it was proposed as one of the solutions to the issue of Greek national debt, them going back to the lira, it didn't go through for many reasons but it definitely wasn't because "there's no legal way". There absolutely is.