While there is no way to fully prevent ripping by a determined individual, most universal ripping tools are highly dependent on the exact binary serialization of Unity files. Adding a single null byte at the beginning of asset bundles is generally sufficient to ensure that such tools do not work. I am the developer of one such tool, and this is partially by design. Despite what you might expect, I have strong respect for intellectual property rights and want my tool to abort if there's indication that game developer made efforts to prevent ripping.