How to remove or reduce pimples on the back?..

Try to use a "Sulfur soap" or any germicidal bath soap. If symptoms persist consult a doctor.

Well, I do not have any personal experience regarding this, but I do know of someone who had this problem, and he said that it really helped him to expose his back to the sun as much as possible, since it dries up pimples. That only works if you are a guy though, I guess... But that is something you could try, if you are a guy and have the opportunity (if you live in a sunny country).

You must have someone to lightly and gently scrub your back with soap and face towel soaked in water. Too harsh might damage your skin even more. Rinse it fully with warm water so that oil and dirt will be washed away. Stay away from the sun. Get more than 8 hours of sleep. Try to avoid perspiration and drink 12 glasses of water everyday to flush out toxins.

something that may be causing it is hair conditioner. Conditioner has tons of oils, so when you put it in your hair, make sure to wash it off your back with soap and water in the shower. also make sure it is throughly dry when you get out of the shower. and when you play sports, maybe try and wipe your sweat away with a warm washcloth and some mild soap.