Terms & Conditions need to be a 'living document' updated when there's a need to. All Discos should have one.
I recall back in 2007 when the smoking ban became law, some clients ignored it for functions on private land such as marquees. I altered my T&C and worked with the clients and had no problems. But the 'belt and braces' approach i've found helpful and have served me well.
By including a clause in my T&C I am not only protecting myself but as you rightly say also my clients. In fact, I've taken many future bookings since lockdown where the client has sought my reassurance that I won't following them around the venue v
The point is, that the client signs the contract, and the behaviour of guests according to the contract remains their responsibility, the same time, threatening or abusive behaviour would also breach the T&C's. I've ended 3 functions over my 37 years of DJ'ing, 2 for abusive behaviour and the other 1 breached health and safety of my guests.
Social distancing is here for some time to come, and I think the deliberate breaches to comply with the law/guidelines would be more than reasonable to stop any event. Communication to the stakeholders is key both beforehand and at the event to avoid unnecessary issues, and for us to mitigate issues such as using a stage where possible, stating that we will take requests by text, marking out an area to comply with the law/guidance.