Rotors out of round?
Type and year of car would help.
It is normal for caliper brakes to maintain very slight contact between the pads and the rotor. If that isn't what you're referring to, about the only things that would prevent the caliper from releasing the rotor would be either a seized caliper piston or possibly a bad proportioning valve.
Front caliper piston seals are gone can get kit for 35 bucks
if your looking for the front abs sensors thay are located in the hubs, and would have to remove front tire, brakes,caliper, and rotor to remove.
Sticky/stuck caliper slides, sticky/stuck caliper piston, collapsed brake hose, pinched brake line,
caliper froze, brake line collapse, or wheel bearing out.
Frozen brake caliper
sounds like your brake caliper is sticking. This would cause the pads to be in constant contact with the rotor.
If you can't compress the piston with a C clamp back in to position to fit the new brake pads I would suggest replacing the caliper completely.
STICKING BRAKE CALIPER
this genereally means that the left front caliper is siezed. if it pulls to the right then your right side is working fine, its the left you need to worry about. as a rule when it comes to brakes, what you do to one side you do to the other. so if you replace the left caliper, then i would suggest to do both of them. and don't forget to bleed the brakes.
Brakes dragging? Emergency brake not releasing? Rear brakes not adjusted properly? Frozen front caliper? Bad "U" Joint? Bad master cylinder- not allowing fluid to return to resevoir?