What does it mean when you dream of a spider bitting your ex?

The spider in this dream certainly represents some form of vicious harm, possibly in female form (think of the Black Widow spider.) But the interpretation hinges on the emotions experienced by the dreamer. This might be a wish-fulfilling dream about pay-back, expressing your suppressed (and possibly justified) desire that this ex will experience pain and suffering. Or, if the dream is anxious and fearful, it could mean that you are overly concerned for your ex's welfare, still caught up in a web of concern that anyone other than you would hurt your ex. In that case, it is showing your continued emotional attachment to this person, and the need to cut ties (clinging webs) and move on with your own life.