Sounds like a bunch of BS....sorry.
Africans dig. It is what they do. Which is why you'll find that most african setups do not include plants - especially when it comes to mbuna, which is what I assume you have. You can't change their behavior, they're going to do it basically no matter what you do - void of actually stripping their tank entirely.
It also doesn't mean they are getting ready to breed. It means they're alive. Again, assuming you have mbuna (or even haps or peacocks, also from Lake Malawi), they don't build nests. Females hold eggs/babies in their mouths, there is absolutely no need for a nest. They just dig as they see fit, regardless of male or female, species, age, breeding or no...
Basically what I'm saying is, you had might as well get used to it
If you're really so determined, you can do as Matt suggested and tie or weight down the fake plants so that hopefully they'll stay in place even as the fish dig. That's about all you can do.