plotting - How can I show constraints on parameters in 3D plot?

How can I exclude a region in a 3D plot?

I have done it for a Density plot, but for the 3D case, it seems difficult to exclude a region (with a certain relation between two main parameters). In fact, the 3D plot is for a relation in terms of "a" and "b", and each one of these parameters has a limitation. I want to denote the area of banned values of "a" and "b" (which is an inequality relation between these parameters) by a red boundary in the main 3D plot.