If you do not want your clients to select specific times you can create time slots for them to choose from. For instance you can create a morning time slot from 8 AM to 10 AM.

In order to create time slots go to the Settings menu and select Time Slots

Click on the Red + button on the top right of the table and you will get a dialog box to create a time block.

Now when the clients log in they will not be able to select an exact time but instead they will have to pick from one of the time slots you have created.