Hi Cellist
A hard case would help, as would keeping the cello well out of sight (mine lives in my study, which is not a room into which visitors would usually go unless I invite them in - I have a six year old son, and I don't want his friends anywhere near the cello! My sewing machine and various other delicate or dangerous items also live there, well out of reach). My family are very well-trained, including both my children - no-one would dare touch the cello other than me. My daughter has her own cello, so understands how fragile the instrument can be and has had considerable experience in telling her friends that it is not a toy

Even my son is very respectful around the cello, bless him, though I still wouldn't leave them alone together........ I think if you are very firm with your friends and family and tell them that it is worth a lot of money and very easily damaged, so not to be touched in your absence, then you can also train them to treat it with respect.
It sounds like there is a certain amount of novelty value to your cello - have you only recently brought it home? It might be worth putting aside some time to formally introduce it to the family and your friends under your supervision (tell them that you are the only one allowed to touch it, demonstrate what it can do, then carefully and lovingly put it away but make a point of doing this yourself and also not allowing anyone else to help.......that might give them the idea that it isn't a toy). Once they have seen it and got used to it, their curiousity may be lessened and so they might be less tempted to touch it. But I think you need to be very very firm about what the rules are and make no exceptions - if they see you taking it very seriously, they are more likely to understand just how important this is. (And explaining how much it cost and how much it would cost to repair might help too.......in my experience, my daughter's classmates often didn't have a clue just how expensive a cello is, but a very firm reminder from me and an icy glare if they were getting too close and too rough seemed to do the trick, along with a comment about how much it was going to cost their parents if they jumped on her cello............

)
Bottom line, it's your cello, you are allowed to set the rules and I would be seriously annoyed if someone was messing around with my cello and damaged it, so I do sympathise.
Wolfnotes