Destination wedding and room bookings

If you haven't already made it clear, I'd send one reminder (text, email, however you communicate best with this group) and let them know that the block is available now, and given that the wedding will be during a busy travel season at your location, you strongly encourage them to book as soon as possible to avoid not being able to stay at this location. Once you do that, I'd let it go. Whether you think they are slackers or not, I'd leave it to your friends/family to take care of their reservations or figure out alternatives at a later date. Unless you're going to pay for the rooms, I don't think you can/should try to manage what they do beyond that. Good luck!

I sent a reminder text to BM & GM once, but that's it. Our save the dates had a small insert card with our main hotel block information too, and they went out 10 months early.

There will always be people who try to book late or at the last second, apparently. I hired a travel agent for this very reason (actually, we are not paying them a dime, but they are assisting with booking our destination wedding). I agree with MOB So Cal's advice to send out a reminder in some fashion to let them know about booking ASAP. This may cause some people who cannot afford to pay right away to drop off your guest list, just an FYI.