If anything, have Gribble suggest PMing instead but how many times have new people come in and not understood the process of authenticating until someone demonstrates it? How often is everyone checking ;;bc,mtgox all at once? Perhaps limit calls in-channel based on activity, but I don't ending them is the answer.
Also, if gribble replies in PM to messages posted in-channel, people that are too lazy to PM gribble are still going to post their commands in-channel and you'll have more people doing it at once. Instead of replying to in-channel commands in PM, it would be better if gribble enforced PMing only for commands it responds to in PM. Replying with "PM me" instead of executing the appropriate command is, I think, the next necessary step if Gribble goes that route.
a lot of good points here. depending on the type of bot gribble is, can you signify two different command thingies, like ;; for in pm me and ! for respond in channel?
also, just as one member of the channel, i find the increasing amount of random bickering about authing in channel much more annoying than any amount of channel spam from the bot