Parents - As punishment, do you think it's ok to withhold something that your teenager bought?

I told my 17 year old that as parents, we are the boss. Regardless if they buy something with their own money, we control what deserves to be used or not. I withheld my son's Ipod Touch for 48 hours as punishment a few weeks ago. It seems to work better when you take something they are proud of obtaining with their own money than by just telling them no tv or what not. It worked for me.