If they gave you a postage-paid envelope in which to return the white ballot, I suggest that you put a little sheet of paper in the envelope that says F U, and then mail the otherwise-empty envelope back to them. It will cost them the return postage.

I know someone who saved junk mail, and then when getting appeals for donations from politicians they didn't like, would "donate" the junk mail to them, in the postage-paid envelopes. What made it especially good was that the envelopes were always pretty heavy so the politicians' campaigns had to pay extra postage.