Keeping a guitar regardless of you trying to collect it.

Loaned a guy a surfboard, precious, he won't give it back. He acts as if he earned it.

He didn't. It's your. Tell him if he won't give it back, you'll get a court order to make him. Guys who want a souvenir to remind them of a relationship usually keep a letter you wrote, not a surfboard.

Because you let them?

If its yours ask them for it. Actually, write a letter of demand asking for it back or you'll take legal action.

They won't be keeping it because it reminds them of you or any rubbish like that. They just want it. End of story.