Example of such filter is in this picture. Red line is output of the filter. Marked in magenta is a period of no work, where credits were given only to bunkerers. Green are problematic areas where slight underpayment would occur.

Now, bunkering is not always evil. Some people have large cache set, or BOINC just decides to download somewhat more workunits. With strict greylist, work done during no work would not be rewarded and profit-inspired crunchers might start aborting wus (which is waste of computational resources, the wus were already started and are valuable for the project).