I purchased an app and found out it cannot be used, how do I get refund and remove that app

You can contact iTunes support and see if they will issue a refund, but they don't have to. The terms of sale is that all sales are final.

to remove it from your device, hold down on the icon until they all start to wiggle. Then tap on the white X in the red circle to delete it. Single tap the home button to exit that mode.

Here's an article that may help, although as stated, most of the time all purchases are final.

http://www.imore.com/how-request-refund-itunes-or-app-store-purchase