spacial aggression/shi tzu

Question
We have a 4 year old neut/male who has been so good until lately.  It seems he feels the need to growl when approached by one of my daughters friends (we have other play dates but only this girl is a prob.).  Its not the girl (very timid and quiet).  He does this also at times for no reason at all with me (the alpha/feeder/mom)when I am putting the kids to bed (he sleeps with my son - but I have stopped that because of this periodic growl).  Never with our kids (ages 8 and 9).  When he does, I firmly tell him "NO" and then swiftly pick him up and  either put him in the bathroom for a while or if its at nite, I put him in the crate overnite.  After I get him out of "time out", he is very meek and timid, and much better, however this hasn't stopped the behavior entirely, only for that day.  Is there anything I can do to stop it?  Am I doing the right thing by isolating him??  If I am, then why does he still (inconsistently) do this?  He is great with food and toys and adults.  Periodically he is aggressive with rawhide chews, but he only gets them maybe 4X a year.  I groom him, not a prob with that either.

Answer
hmmm... seems like he is getting dominant i think the isolating is a good idea but only if you do it as soon as the unwanted behaviour occurs this may sound odd but in a way you want to see the behaviour to correct it before it turns into agression when he does it pick him up with the scruff of the neck for a moment this is the most vulnerable part of the dog and is where the mother would hold him and say no i would suggest placing him in a basket where you can see him but where he can get out because you need to make him stay there without attempting to escaoe then your letting him know what behaviour you want as the pack leader lol. each  time he comes from the basket say no pick him up and return him until eventually he surrenders. try this if not we can look at other techniques but i know you must be worried that this may escalate until the bahaviur has improved do not allow him to sleep with anyone as this encourages dominant behaviour
let me know and good luck