The context in which you want this is unclear. Do you mean like the copyright notice in a theme’s footer, a theme that you’ve developed? Equivalent to the “Powered by WordPress” line that is often in WP default themes?
If so, there’s nothing you can do. The site owner has access and control of their copy of your theme’s source code. Anything you could contrive of to make things difficult could be circumvented.
For example, you could implement a hashing scheme on the file containing the notice. Should the file be altered, the hash check would fail and code could abort further loading. Sounds doable, right? All someone would have to do is comment out the hash check and the entire scheme would fail to work as intended.
You could even cryptologically encode everything so what your hash check is really doing and how to defeat it is obscured. But the decode key has to be available for the code to execute, so all anyone needs to do is use the key to decode everything. You can obfuscate and make things difficult, but you cannot prevent.
