What could you do for tooth decay other than a filling?

Take it out... There are many ways to treat tooth decay. But...If you don't wish a filling or can't afford what is being suggested...Then the only other alternative would be to have the tooth removed. Then you won't have to worry about its care anymore.
Extraction. Those are the choices. If the tooth and bone and bite are all good --- go for the filling. Especially at your young age.
Restore or remove. The only real way to deal with decay is to remove it, or it will continue to grow to the point where pain and/or infection can take over. Removing and restoring decay can be as simple as a filling or more involved as in a root canal or crown. When decay is so great that a tooth cannot be restored, extraction may be the only option.

Related Questions

Is the failure of composite filling usually due to under jaw pressure or from tooth decay?

Composite failure. Composites or tooth colored fillings are chemically bonded to the tooth, failure of the filing may be due to a breakdown of the bonding. There are many things that may cause this including additional tooth decay. Read more...
Composite fillings. Fillings can fail due to fracture or recurrent decay. Periodic check up can keep everything under control. Read more...
Man made materials. . No man made material will last forever. All will break down by erosion and wear, begin to leak at the margin between the filling and the tooth and decay will begin or other situations. In your case my main thought is how long has the restoration been there ?If it has been 7-15 years, i would conclude that the restoration has probaly lasted its life expectancy. Read more...
Both. Can be an issue of overload or torqueing forces which exceed the bond and/or mechanical strength. Decay can also be a factor which provides loosening options for the filling. Often it is both. Read more...
Composite failure . The failure in the first 3-5 years is a restoration issue (e.g. technique or bonding material selection) and after that time period from secondary decay. Read more...