I totally understand this problem. I also have a sofa bed that could be easily damaged through rough or inexperienced guest usage.
My preferred option to prevent that over the years has been to insist on making up the sofa into a bed myself before their arrival, and then guests don't have the option to change it back into a sofa during their stay (the sofa cushions are removed). I put this explicitly into the listing, and I also highlighted in the initial booking correspondence to guests who would be using it to ensure that this aspect was well understood at the outset. If it was unsatisfactory for them then they could of course just cancel their booking straight away without penalty and choose an alternate option.
This has all worked out well for me over the years, and I don't recall losing any bookings or getting any complaints or bad reviews as a result of this limitation, and the sofa bed has remained undamaged.
I have recently however decided not to offer the sofa bed any longer as I would now prefer to offer just the sofa in order to ensure a better overall quality of stay. I also find that some guests have unreasonable expectations that a sofa bed will be of the same comfort level as an ordinary bed, which would rarely be the case due to the inherent mattress limitations.
If you wish to continue offering the use of your sofa bed then I hope that this idea may perhaps be of help to avoid any further inadvertent damage being caused to it. Guests do not usually mind something like this as long as it is clearly explained and agreed with them up front.