Un installing paying app

I installed Boomerang (slow Mo) camera. But it had a free trial period of 4-5 days. I uninstalled the app (at least I thought I did, way before the time was up to start charging my account) and to my surprised the app was still on my phone. (I think I just took it off) but yesterday I received a notification that Boomerang was trying to charge my account. Lucky I never keep money on that account unless I transfer it. My question is, Now that the app has been successfully uninstalled...would they try to keep charging my account and is so...How do i stop it? I think is because I related my card to itunes and App Store....what should I do?