M.G.
It might be a good idea to find out WHY she is doing this... is she a nervous child, is she upset about something, have there been a lot of changes recently that may be disrupting, etc... usually habits like that are related to something other than the actual "act" itself. She's at the age where you could probably talk to her about it. Because it seems like parents just telling children to "stop" doesnt' always work, but finding out WHY they are doing it may help aleviate the problem or at least give you a starting point to begin to fix teh problem. Have you talked to her physician about this? Maybe he/she might have some insight.