Max bookings per block

I am just trying to figure out how to only allow 1 booking per block even when there are resources. We have 5 session types for a customer to choose from but only 1 trainer so cannot allow 1 booking for each session per block.

How do I fix this so that if a customer uses session type 1 for 4:30pm no one else can book that block no matter what other session type they choose. Currently the set up would allow 4 over bookings per block.