I have used and seen before a sign that could throw out text, but not random letters. It moved as fast as the letter randomiser in the wiki, and was very similar, but instead it could repeat symbols that you type. Something like |[-]|[-]|[-]| That's super vague actually, but it was something like that
Uhm you could do &k<text> and I'm pretty sure it will only display the symbols you put in there. So if you do &khadkjslj you'll get a whole ton of random stuff but &k|---| or something would only put random |'s and -'s.
It will only give obfuscated characters equal to number of character put in, so &kbob will only give 3 obfuscated characters.