How can I cut and cap a hot water supply line that has no shut-off after the HWT?
You should have a shut-off valve before your hot water tank (your main entry valve), shut that one off, open all the faucets in your house to drain the hot water from your system, do your repair, turn on the valve that is before your hot water tank and turn off all your faucets. 

You could keep a bucket closeby, just to get what's left in the pipe.