1. Tell them how they should feel
Young adults need to think through difficult issues, not be spoon fed the answers. If you want young adults to flee your church, I encourage you to not allow them to process their feelings, but to rather tell them exactly how they should think and feel about what concerns them. If you really want them to never come back, let them know that how they feel is wrong and unbiblical. You may even want to throw an out of context Bible verse at them...they really hate that.
2. Be unprepared to handle deep theological discussions
Young adults are often interested in discussing the complexities of our faith, especially those who have at least a bit of a background in church. Because young adults want to be defined by who they are and not what they do, they have a healthy craving to discover the nature of God while seeing themselves through the lenses of scripture. Dismiss every longing for deep theology with a soft serve presentation of ‘how-to' pragmatics and I guarantee that young adults will no longer be interested.
3. Have an agenda
One of the best ways to get young adults out of your church is to meet with them while having an agenda. Young adults are very perceptive so you don't even need to be obvious about this. Call that great 23 year old guitar player for the sole purpose of asking him to play on the worship team Sunday morning. By all means, don't be interested in him, just be interested in what he can do for you.
4. Be dishonest
Young adults place a high value on honesty and bluntness. They will know you love them when you can gracefully be straight forward and honest with them. If you want to completely fail with young adults, I strongly encourage you to be unwilling to have difficult and honest conversations with them. They need and want someone to be straight forward with them about their spiritual journey. If you are reluctant to go down that road with them in a loving, yet confrontational way, they will feel unchallenged and unloved.
5. Be uneducated and disinterested about social issues
Young adults want to make a difference in this world and want to make a difference now. They know about the pain and suffering taking place in this world and truly believe that they can do something about it. If your church is doing nothing about poverty, human trafficking, orphans, widows, and other social issues...your church is completely irrelevant. A young adult couldn't care less about a church that doesn't care. While you complain about young adults leaving, they are giving their time to help restore the broken things in this world.