First of all: Nothing can be made “undeletable” in things you can display in a browser. It’s only undeletable for people who don’t know the right places in which to look.
Now, that said, you can certainly make it harder for people to remove such notification but it depends on what kind of set-up you have. If you have plain HTML you can hide the notice by encoding the text as HTML entities. If you have some kind of server side language you can obfuscate the text (e. g. by hex-encoding it) and include it in an obfuscated function itself. That’s where a lot of the spam in free Wordpress themes comes from.
The most effective way is to use the law. Specify in the licence that it is only to be removed upon payment of an additional fee and if anyone removes it without paying that fee you then threaten legal action. If that doesn't resolve things then you proceed with the legal action to protect your copyright.