unprofessional behaviour of cartack company and harrassment they are subjecting me to

Cartrack increased my premiums in 2017 3 times within a period of 6 months....i cancelled them and terminated my contract with them because of that. They have been doing this to other clients as well... Now they keep sending me silly messages telling mw i owe them when i terminated my contract with them i had to ho around looking for another tracking company.....please assist me they are harrassing me.