Hi, another retail person here.
Sensormatic has a lot of different systems now.
If it is an INK tag it will say so on the large tag portion.
If it is the sensormatic tag, they can be ivory or gray paddles that are snapped on with a thumbtack device on the inside. With these somtimes the clerk does not push them together as tight as they should and you may be able to get a small wire cutter in between the tack and the clothing.
Another option. Most times, not all, the tag is inserted into a seam that you may able to open using a seam ripper and then sew back up.
If it is the ink type you can cut the tack portion also BUT you will need to cover the ink portion very well as once the tack is cut the vials of ink will break open onto the exposed clothing.
I know, this is all a big pain in the butt!! good luck....