How can I set my sub's gag to not allow outgoing IMs, except to me (and to a friend)?

The default RLV setting is for all outgoing IMs to be blocked.

If, however, your sub has attained the circle of Novice and above, a new feature is unlocked - her owner can choose to have the gag "garble loose" (on the gag menu).

When the gag is set to garble loose, outgoing IMs are restricted, except to the following:
The IM Exception list will not work unless the sub is Novice and above.

If your sub is still at Initiate, the only way for your sub to send out IMs is to turn off the No-IM RLV feature (on the gag menu, choose "other...",  "RLV options...", and uncheck "No-IM").