One of the rules we had in a group I played with was similar to the idea Valegrim suggested, except that instead of mock talking with your hand, you had to hold a pen or pencil above your head to go OOC. XP was lost if you didn't adhere to the rule, or perhaps even worse, if the DM was feeling creative.
The idea I thought of while reading this post was, during one of the times the other players are talking over your one role player, continue the scenario with her. Make it something short and easy, talking to an NPC or something, but give her an entire level for it. If this is contested, tell your other players that it was meant to be something that everyone participated in and split up the exp, but they weren't playing their characters and were too busy OOC and were not involved in the conversation or something, so your one role player got all of it. That may or may not work, but it is just something that came to mind. Having the role player be a level ahead of everyone could potentially serve as a constant reminder to the benefits of role playing.