what is the easiest product that I can use on my dads black 2000 SE to cover up rust and protect that area from future corosion?

por-15 is a good product for rust. but in the end nothing ever fixes rust, it just slows it down.

yup, the only true real way to stop it.. is by replacing the infected metal

That depends a lot on how pitted the rust is into the metal. If it is pitted, I have found sand-blasting to be the best way to prevent the rust from returning. Even a cheap siphen feed ($10-$20)will work great for small areas. Since the early 80'*, I have tried sanding, Rustoleum primer/&paint, and several of the "rust converters" on the market. Even Por-15 dosent seem to stop rust from returning after several (4-7) years although it does better than the other coatings. Get the rust out first by blasting it, thats the best advise I can give. Once you try a sand blaster, you will never think of sanding rust again. Just be careful not to blast the same area too long or you will thin the metal and possibly warp the steel. Also, use duct tape and poly to section the area off from the rest of the vehicle. With-out a "hooded cover" over the area, the sand will get everywhere. Also, tape over the bottom window seals to prevent sand from getting in and scratching the glass when you roll down your windows. And wipe off your windshield and wiper blades for the same reason.

the rust spot is about the size of a quarter and its on the inside of the back door

in that case i would either sand blast it like pontiholic said or try sanding it off and then painting it

could I put rustoleum on there?

I'm not sure how well that'd hold up :?

you can put anything that you want on it. rustoleum will work but wont be a permanent solution, but it should look fine though

IMO, spend the extra dough and get it done professionally. I've seen too many cars around here with BAD bondo work, and it'* like they just primered it and forgot about it... So yeah, get it done professionally