Disable dump()?

The closest you can get is global.dump=function(){};E.dumpStr=function(){};, but there are ways around that. Also if someone has command-line access to Espruino then they may well be able to pull something out.

Probably your best bet is one of:

• E.setPassword to put a password lock on the REPL
  
• Disable the REPL completely - I'm not sure what board you're using but if you let me know I could give you some advice.
  
• Minify your code, which will make it pretty unreadable. You could go further and use some other tools to really try and obscure your code if you want to.