Writing 0xcc to RDP option byte makes it possible.
But it doesn't allow you(anyone) to use JTAG anymore.
So please use it very carefully.
For detail, please see programming manual of STM32.
Thank you for replying!
If I use this method, can I still over-write the firmware using the JTAG and OpenOCD?
0xcc is the nuclear option. It isn't reversible. The Level 1 is probably more along the lines of what you are looking for.
Here is a more detailed answer, but as dmiya said, the STM32 programming manual has all the details.