What worked for me was using Alamo's automated system through their customer service number. I just selected the cancel option, entered my confirmation number, and the automated system canceled it. I did call back a little later after not receiving any confirmation email and spoke with an agent, who assured me the reservation showed as canceled in their system. The next day that reservation was gone from my upcoming trips. Hopefully one of these options works for you.
Thank you. This worked for me.