No, no, never would do this. If, as a company hosting an event, I want them to enjoy themselves and then spread these pictures of their own accord. Blocking access to the pictures to get feedback means you aren't getting accurate feedback with or without blocking.
* gives event planners data points to attract more/higher quality sponsors
Imagine what your local dev meetup organizers could do if they had more funding (free arduinos? Fly out Woz? Hand out AWS credits?)
I designed the feedback form to be completeable in under 15 seconds, while giving good data to event planners. You don’t even need to provide your name. I don’t think it’s unreasonable for an event planner to ask 3 questions in order to provide some photos - photos you can use to build your personal brand, post on dating sites, cherish as memories, etc.