You probably figured this out by now, and it is a printer question rather than Thunderbird specifically. For other's reference, a certain amount of the current print process will have loaded into the printer memory, and more of it will be 'spooled' in Windows ready to load. This is what keeps printing after you cancel the print job in Windows. You can empty the spooler service - look for articles explaining how to clear print spooler service. Also clear the printer memory - you may have a way to do that through an interface, or if not, unplug the printer and plug it in again, wait to see if it starts printing, and repeat that until it does not continue printing. Each off will empty memory, on will load the next batch, then empty when turning it off and so on. So if the print task is not too large, just shutting down the printer may be enough without clearing the spooler.