So glad there’s a term for this:
In the legal profession and courts, a querulant (from the Latin querulus – “complaining”) is a person who obsessively feels wronged, particularly about minor causes of action. In particular the term is used for those who repeatedly petition authorities or pursue legal actions based on manifestly unfounded grounds. These applications include in particular complaints about petty offenses.
Querulant behavior is to be distinguished from either the obsessive pursuit of justice regarding major injustices, or the proportionate, reasonable, pursuit of justice regarding minor grievances. According to Mullen and Lester, the life of the querulant individual becomes consumed by their personal pursuit of justice in relation to minor grievances.
Hat Tip Ratcake over at Kiwiblog.