Can I manually block out dates so a vehicle can't be booked?

Yes. Open the Fleet Calendar, choose Add hold, pick the vehicle and the date range, and set a reason (maintenance, owner use, photo shoot, damage, out of service, and so on). Those dates go unavailable straight away — no booking required.

For repairs there's also an off-road option on the maintenance side, which takes the vehicle out for the length of the job.

Holds are per vehicle, so to close out several vehicles for the same dates you add a hold to each.