Answer
You could protect the Paramecia by giving them a place to hide, or you could add other organisms, either ones which Didinium prefers to eat or which would prey on Didinium itself.
Work Step by Step
The key here is to see that Didinium is driving the extinction of Paramecium, so something must be done either to control that interaction in favor of the prey or to alter the ability of Didinium to prey specifically on Paramecium.
