Adam Engst, writing for TidBits:

In essence, Apple stores the Find My Mac data in NVRAM, which is good for keeping it around even if the hard drive is removed, but bad in the sense that it’s easy to reset NVRAM — just restart while holding down Command-Option-P-R. A quick test confirmed the problem in OS X 10.11 El Capitan, and nothing has changed in the public beta of macOS 10.12 Sierra.

The only way to prevent this is to set a firmware password.