If You need something obvious, jinx them ! Cause them badluck, and accidents to happen, though limit the spell/ritual's endurance. That should make it tad more fair.
You could combine Guinea pepper, with Valerian root and Ground Ivy, and some D.shoestrings, make it into powder, over which You spoke Your intentions to cause someone to trip, fall, lose stuff etc. and "throw it behind them" ( conjure method of crossin g ) , or lay some of the powder in their footsteps in the ground, abusing their name three times etc.