There are a few possible reasons in which a string trimmer will continuously feed line.

Any type of lubricant, debris or dirt inside the outer spool area may interfere with the friction teeth and/or return spring disabling, depending on the exact head design, the spools ability to stop line feed. The indexing teeth inside the inner reel components may also be worn, which can cause the line to slip and feed without tapping on the bump knob.

NOTE:  Do not apply lubricant on these components as this will likely prevent the friction teeth from working properly. For both situations, make sure these components are clean and dry. If the issue persists, replacement of components is likely needed.