It is very possible to use 9-speed shifters with 7 speed cassettes. There are two ways to do it.
1. You could route the cable differently on the derailleur, as explained
here. Basically, follow routing "B" which works for a 9-speed shifter with an 8-speed cassette. Since 7 and 8 are so close to each other, it will work.
2. Install
ShiftMate #2.
Method one is cheaper but a bit tricker to install. And since the cable has a little more tension, you will need to adjust indexing a little more often.