Yoma 87 explains how to properly ask for forgiveness. The offender must go with three friends to the offended, and publicly ask for forgiveness.
If the offended does not want to grant forgiveness, this process is repeated a second, and if needed, a third time.
After the third time, the offender need not ask for forgiveness again - and the offended has transgressed the prohibition of holding a grudge!
So - if the person who is repeatedly transgressing against you isn't admitting his sin in front of friends and asking you for forgiveness, you don't have to forgive him. If this person is not well versed in Gemara, explain that this public admission and sincere request for forgiveness is what you need in order to forgive, and nothing less.
(If the person is willing to shame him/herself publicly by admitting the trangression and publicly asking in a contrite way for forgiveness, that's a fairly decent indication that the person really regrets his/her actions).