You put your own reputation on the line when you write a letter of recommendation. If you feel you do not know the person well enough, or they are somebody you cannot in good conscience recommend, decline their request.
Close the letter affirmatively. Reiterate your recommendation of the candidate and, if appropriate, invite the recipient to contact you. For example, say, "For all of these reasons, I think Michael will make a fine addition to your team. Should you have any questions, I invite you to contact me at the number or address, above