Stop all work until they pay up. I learned this the hard way after getting burned too many times. When they call with urgent work, I tell them I’m booked solid until they settle their outstanding invoices. Most clients suddenly find money when their project’s stuck. The ones who don’t pay weren’t going to anyway. Your cash flow matters more than their emergency.
Tell them straight up - no new work until they pay what they owe. Don’t overcomplicate it.
I usually say something like: “I’d love to help with this urgent project, but I need you to get current on the outstanding balance first.” Works most of the time since they actually need the work done.
Had this exact problem two years ago - client kept demanding rush jobs while sitting on three unpaid invoices.
I started requiring payment on outstanding invoices before taking any new work. Just told them “I need to get caught up on open invoices before I can take this project.” Most paid up immediately.
For pushback, I’d do the urgent work but required upfront payment for the new project plus a payment plan for old invoices. “I can do this rush job if you pay now and we set up payments for the outstanding balance.”
Lost one client over this but they weren’t going to pay anyway. Everyone else respected the boundary and our relationships actually improved.