How to delete an app that says it's "still open"?

I downloaded Stamps.com app for a trial. I noticed that it didn't have an uninstall. I highlighted all relevant items to stamps.com including the app and dragged them to the trash. I got the error message that the app can't be trashed because it is still open. So it is running in the background as I've tried to reopen the app and nothing happened. I tried to highlight it and force quit it but a "force quit" didn't appear. I'm on an iMac with El Capitain! v. 10.11.2. Do I need to redownload their program to see if there is an uninstall and do it that way? Or is there another way? Thanks, Learning77