You need one of the pro grade network hubs that run a few thousand and then you create VLANs. Frankly why make their network your problem. I've done this before and supply a complimentary internet access but never solve their network issues.
If those companies want their own LANs then tell them to make their own.
I have an office with 3 tenants. How can I seperate their networks without havign to buy 3 routers?