Let’s say there would be two thresholds, for instance 80% and 30%, and also a max limit (and maybe a minimum limit too).
If a belt would be mined beyond the upper threshold, the next time it spawns, it would spawn less minerals (depending on how high it was overmined). Any time it is under the lower threshold (but equal or more than 1%), it would increase the difference between the mining amount and the threshold. Note that if it is between the two, it stays so.
So if a corp would need lots of minerals to replenish their stocks fast, they would be able to overmine, but that would reduce the amount next time.
Also, if enemy Prospects or other miners would sneak into a region inhabited by their counterparts, they would be able to ninja mine and actually damage the economy of the other corps by decreasing their gain from the belts. That would be an offensive way to use miners, greatly enhancing their role in wars, and as such, it would result in a more interesting gameplay and new options for corps and alliances to consider.