Hi bredmond.
The "Prepare toner cartridge" message indicates that the toner is starting to run low and it is recommended that you have another cartridge on hand, to avoid loss of production, when the current cartridge is completely empty.
You should still be able to continue to use the printer while this message is displayed. The only way to disable the message is to replace the cartridge.
If you have any other questions, or require further assistance, please call us at 1-800-OK-CANON (652-2666), Monday - Friday 10:00 a.m. to 6:30 p.m. ET (excluding holidays).