The spa should have set flush with the pool wall to avoid this problem.

We have used a custom extension of copper or brass that has sides to prevent spillage to the side. www.oregcorp.com Custom Cascades is the model.

These are custom fabricated so if you decide to order, measure carefully. The lip should extend past the edge of the coping stone at least 2". On windy days it will most likely blow back. And yes, swimmers must be careful of hitting the edge when surfacing.
We will only set a spa like this when we install an automatic cover and need a spot to install the hidden track.