Can a disciple kick his mentor?
Yes. The disciple has a grace period (10% of his disciple commitment), within which he/she can regret the decision to accept a person as his/her mentor. This time limit is connected to the disciples skill progress. After the disciple has reached a certain level of skills, he/she cannot revoke the mentorship.

My mentor is useless. How can I remove him/her?
Once you have accepted the mentor and you have passed the grace period, in which you are still able to kick your mentor, you cannot leave the mentorship program until your mentor has released you.

My disciple has completed his training. How do i collect the mentor gift?
By pressing the K key and reviewing the the AVATAR INFO, you can see the progress of your disciples, there is a treasure icon next to the name of the disciple who just graduated. Press that icon and you will receive your reward.