Forums Scripts & Popups On Join Action For Specific Person

Nope, this isn't restricted in that a way.

You may have
a) some other "on *:JOIN:#JohnDoeChannel: {" event loaded, using a /halt command
or
b) another on *:JOIN:#JohnDoeChannel: {" event loaded in the same scriptfile

To greet that specific nick, extend the "describe $chan <your described text here>" command, e.g.:
describe $chan waves Hi to $nick $+ !