Diagnosis
The pulse button's tactile switch on the control board is stuck in the closed (on) position. This is a physical failure of the switch component.
How to Fix It
Unplug the blender. Firmly press the pulse button several times to see if it frees up. If not, the control board must be accessed and the faulty micro-switch replaced or the entire board swapped.
If That Didn't Work
The issue could be software. Unplug the blender for 5 full minutes. This resets the main control chip. Plug it back in and test. If the pulse still sticks, it's hardware.
Heads Up
Do not continue using a stuck pulse function. It indicates a failing control system that could lead to other erratic behavior or a failure to turn off. Consider it a major control fault.