Nancy Beach has a post about a colleague that believed they needed to ask a Worship Band member to step down in order to take the team to the “next level.” I’ve been there.
To start, there are 2 main things I look for when selecting new Worship Band members. First there needs to be a heart attitude of worship, and a desire to be a servant. Second there needs to be clear gifting in the area they desire to serve in. Both of these things are necessary. Sometimes we invite someone into the Band when only one of these qualities is developed, and use the opportunity to move them along where they are lacking. Sometimes it isn’t immediately obvious that they have what it takes in both these areas. When you’re not sure, you have to go slow. Ask them to come to rehearsals, but not to lead on Sundays. For vocalists, ask them to sing in the choir for a few months before singing on a mic up front. See how that rubs them. Their reaction (and whether they even stick around) will clue you into where their heart is.
But what about someone who is not up to par in the musical/technical side. Here are my suggestions in handling that:
1. Let them know the expectations. Tell them you expect a degree of excellence in the presentation of music, and this requires practice apart from band rehearsal. Bottom line – a musical mistake equals a potential distraction in people connecting with God in worship.
2. Be specific. If it is a stylistic problem, tell them how you want it. If it is a technique problem, give helpful suggestions how to improve. Offer to help via lessons outside rehearsal. You can’t just tell them it’s not good enough and not provide a chance to correct it and grow.
3. Put the burden on them. Let them know that unless they get help and improve, we don’t have a place for you to serve here…
4. Offer another venue for them to continue to serve and improve in. There are usually other places that need musical leadership: children’s church, youth band, small groups, prison ministry, etc. Let them serve somewhere else.
In the end, a band is only as good as it’s worst player. If efforts are made to improve and nothing is working, you have to cut the cord. Tell them, “We love you, but God has not gifted you to serve in this area. We know He has somewhere else for you to serve here.”
ClaytonFaulkner.com 
Harsh but fair. It is always difficult to cut someone from a music ministry, I think it is important though. (a chain is only as strong as its weakest link) I beleive that if someone takes it really rough then their attitude is not that of a worshipper. If someone truly believes they are called to be a worshipper then they will try to get better and then come back. At the end of the day, many people will come along that are not ready to join the ministry but think they are. It is our job as pastors and leaders to make sure that we talk to these people before an offense rises. It is better to be honest from the get go than just sweeping things under the rug. Good Advice, thank you.