Are you the owner? If so, then they can only let themselves in with your permission.
If it's rented, check your contract. There is often a clause allowing them to show people round in the last few weeks before you move out, but they should at least notify you beforehand.
No im not a home owner, iv been renting for 6 months. I dont feel too happy about people coming while nobody is there.
I know this will sound irrational but I have a problem trusting people, iv had a lot of my thing stolen off me before so its made me very wary!!
Our contract says that we "must allow the letting agent and potential tenants access to view at reasonable times and with 24 hours notice".
I know when we wanted to view the place before we moved in, the agent wouldn't just take us there, they said they had to make sure the current tenant was in at the time.
They have to give you at least 24 hrs notice but once they have they can let themselves in. You could always offer to pay an extra months rent when you move out so that they can find a new tenant then.
At the end of the day both you and them have to be reasonable about it.