How can I control the pulling power of a DC motor?
Typically in a motor current is proportional to torque, so you need to have a motor controller that can deliver a controlled current.

Whether or not it is bad to have the motor engaged and not moving depends on the motor and how much current you are pushing through it while stalled. You could easily push too much power into it and overheat it if you do not control current to a reasonable level based on how much power the motor can dissipate.