Silly things like wearing "management" clothes - business-like suit, hair, shoes and make-up - add to the general aura of being in authority, and somehow filter through to the children. Also stress-relief techniques can help for you. I'm not suggesting that you are stressed up but having a few techniques to "remove" yourself emotionally can help you enormously when one or two children are messing up - it helps me to focus on the children who are getting on sensibly, rather than be p**d off generally.
I teach a really grotty bottom maths set every day at eleven, which used to sit like a huge lump in the middle of the day, overshadowing everything else. All of the children have special needs and I have 4 with behaviour statements. Now, at the end of a lesson, instead of dismissing them at once, I quietly put a smiley face on the board and silently write names under it, for children who have worked hard/quietly/answered lots of questions. Children can go as I write their name on the board, but I don't start unless I have absolute silence and I won't put anyone on the board if they are talking.
Children who aren't on the list remain in class and have to work in silence for twenty minutes before they are allowed to go to lunch. I use that time for planning and marking.
The children who have worked hard love being able to leave a couple of minutes early because they get first choice at lunch/the best tables and those who stay behind hate it because they get the dregs/can't have lunch with their friends. If your HT won't allow you to let children out a couple of minutes early, then just move the clock in your room forward by a couple of minutes so that the children think they're having a treat!
I do it every time without fail and the children are now working OK and the lesson is, from time to time, actually enjoyable! Even the recalcitrant one or two are now usually getting through the lesson without any major hiccups. Often no-one needs to stay behind.
I also do the raffle thing with my own class - I have spies who report back to me on the class' behaviour with other teachers/TAs/dinner ladies and anyone who hasn't had a verbal warning gets a raffle ticket for a weekly draw. If no-one from my class has had to be kept in or had detention then we draw out three prizes. Otherwise we draw out one.
Incidentally, what do other teachers say about this child?
I would say that if your one child contintues to disrupt the lesson and prevent other children from learning, then you need to go down the exclusion route. Your behaviour policy must include a process for escalating problems to SMT/HT level, and for exclusion. This policy must have been ratified by governors. If the policy isn't being put into practice by SMT then they are not following the governors' instructions, so you should go to your teacher governor and have it raised at a main governors' meeting. If it is nothing to do with your own practice (and I'm not suggesting for one moment that it might be), then other teachers will be having the same problems and something needs to be done to solve them.