Is it possible to ensure my clients can only book into my sessions by using an active service?
Yes, you can do that by using the Services Only option.
In order to do so, you just need to open a session, Click the edit button, scroll down a bit, and in the payment section, choose the Services Only option and accept the services that you want your clients to use to book into that session.
Please watch the video below that runs through the above steps.
You can also create new sessions and turn on the Services Only option.