Not much you can do with the UV degrading PU based stuff (like Sikaflex 291, which is just plain wrong for the job). Scrubbing it off will help briefly but it will just expose a fresh layer to the sun to turn crumbly. Cutting it down with a blade and applying UV resistant sealant over the top is the proper fix (assuming you want to keep the teak deck for a while).
If you want it to last, use a deck caulking product, such as TBS SIS-440.