Ok, here's my take on it. He was disrespectful for assuming you wouldn't mind if he rode your bike. But he was respectful enough to return it immediately when you messaged him. I think you should first make it clear (without being an ass) that you simply don't want your bike being ridden while you're away. Explain about the insurance and the fact that you just got the bike fixed. You don't want it wrecked and your insurance won't cover him (it would, but he probably doesn't know that). Try to make it sound like it's in his best interest to NOT ride the bike. Explain that you only have liability and you wouldn't want him owing you to replace the bike if it's wrecked. Or just say that even though he's your friend, you simply don't want anyone to ride your bike. If he understands this, then all is good. If he doesn't, then it's definitely time to find a new friend. But keep in mind, true friends are harder to find than new bikes.