I get sick of candy more quickly than I expected. The portion my machine emits (about a small handful) tends to stop motivating me after about 4 in a day. Additionally, I seem to be entirely incapable of pacing myself; if the reward is in the system, I tend not to wait very long before using it. This has crippled all of the rules about involving taking away rewards - unless the rewards are blocked, they don't stick around in the system long enough to be taken away.

All of these problems could probably be fixed by denominating rewards in Quirrel points instead of house points. ;-) (HPMOR reference)

That is, have the device display (you said it had a display) a count of points, which has to reach a certain level before the reward is dispensed. Ideally, make it flash and ring a bell or make a sound for each point as well.

Since you will be using smaller point denominations, this means you'll also want to break some of your larger tasks into smaller pieces, so you can then fix this problem:

if I think a task will take me more than a day to finish, that's more than a day of work which earns me no short-term rewards

For example, you could break that task down into pomodoro-estimable units, then give yourself a point per pomodoro or something. Ideally, calibrated so that it takes you the whole day to get all four dispensings.

If you work a 16-pomodoro day, for example, then four pomodoros could be worth one dispensing. This would certainly increase motivation to not interrupt those later pomodoros in each 2-hour work/rest/eat cycle. (The first one in each cycle you'd not have much to lose, but that should also be when you're generally freshest due to having rested or just starting for the day.)