There are two rather simple ways to enable readback protection using the APPROTECT register. Either, you can set it in your application code like my colleague explains in this thread. Or, you can download nrfjprogon your computer and enable APPROTECT by calling the command nrfjprog --rbp all in a command window when your device is connected. In both cases, you will have to recover the chip/ do a full chip erase in order to access the memory of the chip.
When you set the APPROTECT register, JTAG will keep working until you reset the device, at which point the readback protection will kick in, and your JTAG and SWD will be disabled, and you won't be able to use the RTT viewer.
