Hi Wideawake,
When Steven, now almost 20, was in high school and living here full time I also did what Marti does. Steven loves the computer. Steven had to complete homework or... no computer. I even got to the point that I would hide the power cord until he produced the homework. He did the fastest best homework that way! Also, I used lack of computer time for discipline if he disobeyed me. It was the only thing he cared about. So... it was really the only thing I could use. I found it highly successful.
However, once when I left him (age 17) at home and he had the day off and I told him he had to complete a list of homework AND back chores by the time I got home or he would have no computer that day... he had saved up money and walked to the store, bought a new power cord, and was online all day, LOL!!! It wasn't funny at the time, but, in looking back, it kinda is to me.