terminal service users bumping other

I need to stop terminal service users bumping other users off I have a odd setup
I have the server set in device mode and have set up design1, design2, design3,ext1to 3& con 1to3 I have 25 people and they need to log in to the terminal server for about 2 to 10min?s 2 or 3 times a day. I do not want to buy 25 user licence, that is why just the 10 licences,. What is happening is that say design 1 is on I want to stop someone else logging in to design1 and bumping the first user off, but make them go to design2 & if all 3 devices are used up they will have to wait and try again later on.
How can I set this up please? is it possible?

PS sorry for the bad England words