Possible Clues:

• Do without, as in lent
• Vote not to
• Don't indulge - you'll get a black mark!
• Withhold vote
• Refrain from voting or eating, say
• A book mark to steer clear of
• Don't vote, but put a black mark
• A seaman's thanks in how to keep off it
• Give up smoking, say - horrible mark made by tar
• A bishop and saint contrived to avoid vice, maybe