I have now implemented a check that should make users empty their deposits. Since the deposits are made so that people can vote while not online, they are still supposed to remove the deposits once they log back in. Those deposit slots are not meant for permanent storage but for a short-term convenience. I have seen users that have over 30 deposit slots filled with voting rewards.
I opted for the following solution:
You now cannot trade in the shop anymore as long as you occupy more deposit slots that you should have. Example: A Citizen has 1 deposit slot. If you vote 2x, you occupy 2 different deposit slots, one more than you should have. You will not be able to trade on the shop until you at least empty the one excessive deposit slot.