How to prevent visitors from touching your system

They need to learn respect for other people and their belongings. I have a seven year old nephew and he understands not to touch it. I explained what all the equipment does and also how a turntable works. Although before I could explain records, he grabbed a brand new one with his whole hand. One side now had a nice thumb print, the other had four fingerprints. He thought of it as a large CD/DVD, I was mad, but once I explained how to handle records and it's been no problem. Besides, it was a just a record, my nephew is more important.

So, if a seven year old understands it, I would hope your adult friends can understand it. If not, use it as a teaching lesson - what it all does and how to use it.

I used to have a do not touch sign above my turntable.

On the way into the listening room everyone has to pass two full size racks for (electric) power supply and compressed air. Both are covered in meters, dials and lights. I tell them how much energy is controlled in each cabinet, and for some reason they scarcely even want to look at the equipment.

I just told them once and they remember

It's not cool to touch someone else's gear...   My lady won't touch my system but she doesn't have to.  She can pick up my Tablet and play anything she wants 

Years ago we had a party, actually we had three events back to back so I picked up a $50 pair of Klipsch so I could put my Revel stand mounts out of harm's way.   

Wouldn't you know it, one of my ex's friends put his plate full of food on the speaker and started to eat off of it ....   I looked at him and said.  " Really? You are using my speaker as a table?  The dining room is over there "   What a jerk.  So ignorant