Some multi-port manifolds come supplied with 'plugs' for the unused ports.

The best way to plug an unused port in a manifold not supplied with a plug is to cut an appropriate sized tubing into a10 mm section and to plug one end of the tube, allowing the other end to slide over or into the open port. 

There are a number of ways to plug the open end end of tubing. Some options include applying Sylgard, epoxy, glue, hot wax, or by using a heat gun to melt the tube closed.

ML and MM Series manifolds can be blocked using PE-50 tubing, while MP and MPP Series manifolds require PE-160 tubing.