Is there a way to save a clients data so that we don’t have to pay for a deactivated seat?

Of course! You just de-activate them. Just a couple of steps:
Go to Profile > Scroll down to the bottom > De-Activate Client. See picture below.

By doing this, you stop their subscription, stop being charged for them, and keep their data. 

In the future, if they come back, you just go to the 'Inactive Clients' list, and get into their profile. Once inside, same thing you did before, go all the way down to the bottom, and Re-Activate them. That's all!