Pony walls are not responsible for a vertical structural load, but this doesn't mean they don't need to be strong. You don’t want a pony wall that will move if and when someone leans on it.
There are a few methods you can use to reinforce a pony wall. Add a half-inch plywood board to the wall for added support. The threaded rod method involves inserting upside down joist hangers into the framing. The result will add tension strength to the blocking.