App won’t shut down

Best advice for problems with apps not written by Apple is to contact the developer  

How to contact an app developer - Apple Support 

 ... or request a refund

This Apple support Billing Article should facilitate that.

Subscriptions and Billing - Official Apple Support 

Also https://reportaproblem.apple.com 

If you are saying it is still stuck despite my original advice and after uninstalling then unpair the Watch and set up as new watch

Unpair and erase your Apple Watch Unpair and erase your Apple Watch – Apple Support (UK)

and then

Set up your Apple Watch Set up your Apple Watch – Apple Support (UK) - without reinstalling backup.

Force close app stuck on screen. 

Press and hold side button until the Medical ID and SOS screen appears. 

Then press and hold Digital Crown til the clock face appears

 Or 

 if the side button is set up to show recents, press it briefly, find the app, swipe it left, tap the big red X 

 If the app concerned is Compass...

To prevent it returning go to

Watch app, General, Return to clock. The general setting should be 2 minutes

Then for Compass in the list below

in "When in Session" section, 

turn off "Return to app"

Remove the app and leave it removed.