I would try two things:
1) Open Activity Monitor (it's in /Applications/Utilities/). Sort the list of processes by Process Name and see if you see a process called Boom or anything that could likely belong to Boom. If you find it, click Quit Process. Try to empty your trash again.
2) Open System Preferences. Click "Users and Groups". Click on your current user on the left and then click on the "Login Items" tab. If Boom is in the list, click on it an click the minus button to remove it from the list. Reboot, then try to empty your trash again.