I have a few questions, and ideas..
How long have you kept Mbuna?
How much rock do you have in your tank?
After you have kept Mbuna for awhile, and restack your rock structures enough times you will know where they will dig before they do it! I actually start the digging process for them when i restack my rock structure. When you know where they will dig ahead of time you can somewhat control it, or minimize it. They dont usually dig just for fun - its usually to either open a passage or create space for a purpose (spawning?)
What I do is open up the spaces between (or under) rocks so there is enough room for a fish to go in and turn around. Its not often that I see them digging much unless I forget to open up an opening enough for them.