Put about an inch of a disinfectant like Lysol in a large, flat container, like one of those disposable foil baking pans, then either step in it wearing the boots or put the boots in it. Wait 2 minutes then step out/ remove the boots and let them dry. That’s standard protocol exiting a quarantine barn well as entering the USA in certain cases and from certain countries, they do it right in the Customs and Immigration Hall, some countries do it as you exit their country. Wipe the uppers down with a disinfectant wipe or cloth dipped in it and spray the inside.
Dont use bleach unless you are going to pitch them out anyway, it can change the color, create splotches and weaken the stitching. I’d throw some Lysol in the laundry too, maybe wipe down your car drivers seat and floor mat but doubt you need to wash the exterior. Direct sunlight is a good disinfectant for hard surfaces.
If you wore gloves, honestly, I’d just toss them. Likewise any of your grooming tools you might have used on their horses, if any.
No idea what to say to these people who left out the detail the barn you were sitting was hosting strangles. You should be OK with the above precautions, the buggers like a warm, moist environment and don’t survive long out of it.