Kobalt Weed Eater unspools immediately on running
Your problem is most likely related to not getting the spring and spool stops inside the spool housing assembled correctly. A spring is normally used to keep the spool pushed up against a clutch or stop mechanism. An auto feed system disengages the spool from this clutch/stop as the spool is spinning allowing a length of the plastic line to unwind off the spool. In correct operation the spool would only rotate part of a turn and then engage the next stop or clutch position. In your instance the spring may be missing or it is being installed in a manner that pushes the spool permanently away from the clutch/stop mechanism.

From the operator instructions that you linked the line feed mechanism on this trimmer is automatic at the time the unit starts up. It indicates that the line is supposed to come out 1/4 inch at the time the spool starts up. The mechanism that limits the spool advance, which in this case could be some type of spiral engagement mechanism or centrifugal clutch, may be full of dirt or other debris that keeps it from locking the spool in position as the unit spins up. 

So based upon that take a close look at how the spool mechanism is meant to work and then make sure to remove all dirt and debris - including something than may have wound up on the shaft inside the spool housing.