Can a turbo car be converted to non-turbo?

yes but you wil need good mechanical knowledge as you would need to do more than just remove the turbo and exhaust manifold

the look up what is needed to turbo a car and thing of the process in reverse

you wold need to remap the ECU remove turbo, the exaust piping would be changed to turn it in a non turbo and heaps or parts removed and replaced with non turbo parts

i would not advise anyone to turn a turbo in to a non turbo as you can have presure issues with the engine and ECU remaping can be hit and miss