In fairness I think the organsiser is the one at fault.
If you book a room on the understanding that food and drink cannot be brought onto the premises for consumption and that you must purchase from the pub, then thats fair enough.
How many of the 45 people knew that? For them its a working session and just like when you go to work, most people will take their food and drink with them.
Likewise, the owner should have made it clear right from the start on the day that only food and drink purchased on the premises was allowed and enforced it.

Sounds to me that the people at the top on both sides didn't do a very good job.