Is it possible for a grad school applicant to cancel a letter of recommendation once it is submitted? I asked my professor to send a letter of recommendation for me because I worked for him and I did great in his course. He submitted his LOR and I receive the confirmation email from the graduate school. 

However now I am afraid that he might not write a good letter or I could ask a more reliable person to do that. 

Therefore, I am wondering is there any way to cancel your letter of recommendation once it is submitted?
It probably is not possible.  If it were possible, you should not do it because making such a request will imply the letter is negative.