Turn off iMessage on a device at Apple Store?

So I'm really stupid but yesterday my friend and I were goofing off at the apple store and we made some cool art on an iPad. I wanted it so I signed into iMessage to send it to myself and It wouldn't let me sign in and I checked nothing was there so I just left it. Today however that same iPad was receiving all my iMessages and random strangers were responding and it was super frustrating as everyone was calling me asking why i was saying hurtful things. So I tried signing into my Apple ID and I don't remember the stupid security questions I set but They were obvious and somehow It wouldn't accept them so many times Im now blocked out of my own account. Im beyond done with this **** how can I just turn it all off? Im switching to Android anyway.

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