Wild blackberries and raspberries are frequently found in our dune environments. Using glyphosate should work to remove the unruly fruit plants. The cut stump method is probably more effective. You can also sever the stump/crown from the roots. No matter how you go about it retreatment will probably be needed. If you use an herbicide be sure to READ THE LABEL and wear the appropriate personal protective equipment.
I hope this helps,