From your main screen, tap Menu>Settings>Applications>Manage applications>All>Find unwanted application (Slacker)>Force Stop.
You can even clear all data and cache utilized by the unwanted application from this screen to ensure is not taking any memory from your phone.
but remember, it might be built in bloatware because MILK MUSIC was created off slacker, and is part of system agreement.
Those are instructions for an android device specifically a Samsung phone. The OP has an APPLE iPhone. The two operating systems are vastly different.
To the OP, are you paying a monthly subscription for the Slacker service? You will have to cancel the subscription first. Once that is done, long press the app icon until jiggles (or whatever you call it when all the icon twist and shake on the screen). If the app icon had an "X" symbol in the top corner you should be able to remove (uninstall) it from the phone.
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.
Lrclaiborne, we would never want you paying for something that you do not need. May we ask what make/model phone you are using so we may walk you through the steps on removing Slacker for you?