Some kids will pick up mannerism or acts as they grow up – either from peers or from television. It is possible that such acts are done to attract attention or are in reaction to some thought or situation; they may even be an early symptom of a nutritional deficiency (so I have heard).
While such acts may seem innocuous and harmless to begin with – they need to be addressed at the earliest possible – before they become a habit difficult to shake. Besides discussing this with the family doctor / pediatrician to rule out something serious, it is also important to talk to the child to find out the reasons for such behavior and to counsel the child. Ignoring such acts with the hope that they disappear may not work and with the passage of time such mannerism may become ingrained in the mind of the child thus be exhibited subconsciously
So if after repetitive counselling and requests fails and push comes to shove a stern authoritative approach may just do the trick. I am not propagating physical contact but a very serious parent to child conversation establishing boundaries and authorities may be required. Ramifications for not discontinuing the behaviors should also be spelled out to ensure there is no ambiguity.
If this approach is a change in tack from your usual problem solving mechanism, it may be a shock for the child and he may get the message. It may be upsetting for the parent as well, but with a heavy heart one must do what is required for the betterment of the child. Having adopted a particular approach it has to be sustained through such acts, i.e. there cannot be a softening once you decide that the last straw has broken the camel’s back.