Hide "Join Other Network" option

I don't believe you can't hide it, but you can force requiring admin rights to create a network:

sudo /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport en0 prefs RequireAdminIBSS=YES

Change the en0 to the correct Wireless port Identifier if its not en0, and drop the 'sudo' if its being run from Casper.

I am talking about "Join Other Network" not "Create a Network"

Well, thats an available option as well. Just change the "RequireAdminIBSS" to "RequireAdminNetworkChange", like this:

sudo /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport en0 prefs RequireAdminNetworkChange=YES

Of course, you could have also just researched the above 'airport' command to see if it was an option.