Most source tags are AM Tags (Acousto-Magnetic) which are the 1/4” x 1-1/2” long white strips you often see on packages.
When the tags are placed inside the packaging by the manufacturer, they are always placed near the barcode on the box. This is because when the cashier scans the barcode, they also can deactivate the tag inside.
These tags are deactivated by demagnetizing them. The included link is to a demagnetizer that you can find at Home Depot near the screwdrivers.
To deactivate a source tag, all you have to do is rub the side that says “Demagnetize” all over and around the barcode.
To activate a tag (something you shouldn’t need to do), just rub the magnetize side in the same area.
WARNING: If you rub the “magnetize” side on the packaging, it will NOT deactivate the tag.
If the tag is a RF Tag ( the metal coil-looking circuit), this will not deactivate it. Although these tags are much more rare as source tags.