Ultimately, you tell them the truth. It may not be received well, but a lie will always hurt far worse. No matter how you say it, you cannot ultimately control how they receive it, which means that you cannot guarantee that their feelings won't be hurt in the process. If they're mature or do eventually mature, they appreciate your honesty. For starters, you can begin to back off from them a bit (spend less time). Change the tone and nature of your conversations, what you're talking about. If you want them to be in the friend zone, you have to have friend zone conversations. Don't mix and match. Mature people can usually pick up on these changes and adjust to the flow if it's not too sudden. But be careful of assuming that they totally understand why your spending less time with them, especially if you want to keep them as a friend. Sometimes people don't get it or your intentions and the feeling is not mutual. You don't want to burn bridges, but you should begin setting new boundaries and it's always best to clearly communicate that upfront. Be nice about it and if this is a relationship that you want to keep as a friendship, clearly communicate that upfront so that the expectations are set and there are no hard feelings (as best as you can do).