HOW do you get rid of the large flock of grackles which have taken over all your feeder and making a dirty mess of your entire yard and deck?

The key seems to be avoiding putting out bird feeders that they can access. Grackles typically use feeders during a few weeks in spring and fall, during migration. So, when they show up, take the feeders down and put them back up when they leave.

Answer

First: you have to dig them up.

This can be a tiresome process as the feathers and bones will separate quite quickly once they have dug their way into the yard and died.

Second: put 'em in a bag.

This will be a smelly and pretty gross project.

Third: Throw the smelly bag into your neighbors yard.

Voice of experience; make sure the neighbor is not in their yard or anywhere that they can see you. This will insure friendly neighbors and avoid lawsuits.

OR

Depending on your local, you can use fire crackers or special rounds for your shotgun (see cheaperthandirt.com) to scare the live ones off.

OR

As long as you don't mind dead things you can poison them (and every other bird in the vicinity). The main drawback to this is all the dead dogs and scavengers.