why is the ac on my 2003 ford taurus stays on when the heater is on

do you have it on defrost?

no

I don't know what setup you have meaning automatic or manual a/c. manual being knobs to select. automatic being buttons and a display screen. you should be able to turn ac off with a button or turn the knob to a non a/c setting. just turning the knob to heat doesn't make the a/c turn off, you have to do that yourself. then again I don't know what you are and aren't doing so its kinda hard for me to tell you whats going on,

its manual the compresors stays on when i have the heat on and blows cold at idle after car is hot