MT credits, and donating/voting rewards in general represent an uncontrolled influx of currency onto the market. It's comparable to printing more paper money than your nation can back with hard currency (Gold and silver bullion). This deflates the value of your currency in a situation referred to as inflation. The situation, if not checked, will eventually make MineTown into a virtual Wiemar Republic, you'll need a truckload of diamonds just to buy a stack of dirt. It's essentially an unfortunate matter of economics.
There are two easy solutions, one is to simply remove the uncontrolled influx and instead rely on the levels of currency that are currently in circulation. Eventually this would begin to raise the value of currency as it was slowly removed from the server for various reasons. Creating a one way currency exit to counter the one way entrance of currency would also be able to solve the problem; however, it would be difficult to implement in a manner that would remove currency from the market at the same rate it is introduced.
The first option is certainly easier, but could be seen as a strain to the server, as donors would have less reason to donate and voters would have less reason to vote. I would suggest that the currency rewards for donating are removed entirely and instead replaced with additional commands, vault upgrades, etc. This way, donors should still have reason to donate even without currency compensation. In fact, many donors would probably prefer to have additional commands as opposed to the decreasingly valuable MT credit. Voters, instead of receiving credits as compensation, could instead receive small donor packages for a certain number of votes per month, or perhaps a limited use of some command based upon the number of votes. I'm not entirely familiar with the voting system, so someone with more expertise in that area could potentially suggest a better option.
A shorter term solution would be to halve the value of the Credit, and/or halve the number received per vote/donation. This would not cork the leak in the dam, but would help to slow the rate at which the flood is occurring until a better solution is found. One could even change the use of credits to be redeemed for certain building materials or special blocks instead of currency.
Though it would be nice to imagine we can simply continue pouring credits into the market, it will be the death of MineTown to do so. As prices inflate, new players will find it increasingly difficult to obtain the currency needed to purchase any items, leading to frustration and abandonment. What's more, trading with actual currencies of diamond, gold, and iron will become impossible due to the extreme numbers required; an inventory full of diamonds is hardly convenient to carry around, and having this number of diamonds in circulation makes obtaining high-level tools and armors absurdly easy.