I found a way to prevent the Garmin Nuvi from entering PC Sync mode while you are charging it with USB.
Step 1: Turn off your Garmin.
Step 2: Connect it with USB and let it enter PC Sync mode.
Step 3: When it's in PC Sync mode, lock it using the switch (my Garmin Nuvi has it at the top).
Step 4: Disconnect the USB and then connect it again when Garmin isn't in PC Sync mode anymore (when the PC icon disappears and Garmin starts loading).
And that's it! Let me know if it worked for you.