Shoutbox

You can use the wordfilter in Plus! to replace their words with yours (plus > preferences > instant messages > word filter). Add the leet word and set it to be replaced with what you like, set it to "messages you receive" (only you will see the replaced words, they won't).

You can never ever change their preferences (breach of privacy).

If it pisses you off that much why don't you ask them to type in proper english?