The ornamental design for a theftpreventiondevice, as shown and described.
FIG. 1 is a perspective view of a theft prevention device showing my new design;
FIG. 2 is a top view thereof;
FIG. 3 is a left side view thereof, the right side view being a minor image;
FIG. 4 is a bottom view thereof;
FIG. 5 is a front view thereof;
FIG. 6 is a rear view thereof;
FIG. 7 is a perspective view thereof, shown in an assembled state;
FIG. 8 is a top view thereof;
FIG. 9 is a left side view thereof, the right side view being a mirror image;
FIG. 10 is a bottom view thereof;
FIG. 11 is a front view thereof; and,
FIG. 12 is a rear view thereof.
The shade lines adjacent to the shading define the bounds of the claimed design. Environmental features are shown for illustrative purposes only. None of the broken lines form part of the claimed design.