@Noel13....They can Noel! They have rented your space and, provided they abide by your house rules, they can either spend as little or as much time as they like. I understand that difficulties can arise when it comes to security of the premises and how it might affect you should you wish to leave but, unless you have stated on your listing that the premises are only available to guests between certain hours then they have every right to spend as much time as they want there
To my knowledge, Airbnb have no policy on this and cannot provide you with any help....cheers.....Rob