Vexatious

adjective

1. Causing vexation or annoyance; teasing; troublesome.
2. Full of trouble or disquiet
3. (of an action) Commenced for the purpose of giving trouble, without due cause.

   "a vexatious lawsuit"

4. (of a party or entity) In the habit of starting vexatious litigation and therefore liable to have restraints placed on one's ability to access the courts.

   "a vexatious litigant"