[Appointments+] Changing from 60-min to 30-min intervals in Appt+ also affects 60-min services

I have been using Appointments+ on the site for a while. The current service (Racquetball) is 60 minutes, which allows visitors to schedule appointments from 9-10, 10-11, 11-12, etc. We want to add a new service (Pickleball) that's only 30 minutes. However, when I change 60 to 30 in the General Settings, it changes the grid for Racquetball to allow visitors to schedule 9-10, 9:30-10:30, 10-11, 10:30-11:30, etc. It still forces them to choose a 60-minute slot, but I don't want visitors to have those in-between intervals. How do I accomplish this? Thanks!