There is no getting around that in martial arts.
positivly reinforce effort. positive reinforcement for people not getting technique. Getting toweled up in sparring.
Negitive reinforcement if someone just does not what to do what is vital to the sucess of the system you are teaching.
I am not a fan of gym culture where the instructor is constantly fighting his students.
After you drop them on their head the third time. "Good job, I think you're finally figuring out that your takedown defense sucks and maybe you should get off you're fucking ass and start working it more instead of pretending you're sparring; when in actuality you look like you're on the kindergarten play ground learning to play patty cake while wearing fucking gloves."
Something like that?
If it were me running the show I would do less sparring/rolling and more drills and then you can structure it as you see fit. The amount beginners learn sparring with other beginners is limited. If you want to incorporate more free-form practice use a focused drill, ie start in X position, use Y technique to accomplish some goal against your resisting partner.
If they are to spar/roll it would be better if you could match up beginner with instructor or advanced student but I realize this is unlikely in your setting.
Well Omega already beat me to it. But **** it, don't tell them show them. Show them why they need to know how to grapple, show them the folly of snuffing it off.
Sparring as everything goes and you get on in there and take people down at will and **** them up on the ground.
I know getting pinned repeatedly by someone smaller but more skilled than me made me work really hard on my ground work, and getting countered repeatedly when trying to muscle my throws made me work on technique over power. So I imagine that, if it is an option, going out and blanketing the guys who need to work on their grappling (which sounds like everyone) during full MMA sparring would probably make them realize they suck at it... or they will quit and join a kickboxing gym.
You can always just grab a student and initiate a grappling exchange. Force them to roll by (good naturedly-ish) attacking them.
Also, the "start in x position and use Y technique" thing.