you’d want to begin splitting the issue into sections, unlug the USB, this may stop the spindle from spinning, IF NOT then it’s an issue on the controller board. most likely the 12V mosfet that feeds the spindle is shorted and will appear burned or melted solder, this is adjacent the spindle output plug on the controller. this being short COULD also cause the power supply to cycle on and off resulting in the slow, pulsing you are seeing.
that said, there are better sources of help and knowledge out there than this forum. 17,000 3018 enthusiasts over here: SainSmart Genmitsu CNC Users Group | Facebook