How do I erase & stop paying for the MLB app?

Managing auto-renewing subscriptions -

http://support.apple.com/kb/HT4098

Late 2012 Mac mini, OS X El Capitan 10.11.5 beta 2;  Watch, 38 mm silver AL, Watch OS 2.1; iPad Air 2 & iPhone 6+, iOS 9.3.2 beta 2;  Apple Airport Express

stop paying bill I want to stop this service

Try clicking and reading the link in the reply above yours, it tells you how to turn off iTunes subscriptions