Well for me theres two ways I would do that.
1:Sing what I'm bout to say to them(don't do this is its really really seirouse)
2: I will sit down and talk to them about it and tell them straight up but I wont be mean about it.
Sometimes you just have to tell them and either way they will get hurt a little bit
Hopes this helps! If it doesn't feel free to yell at me or something.
Sierrabiersack