The main recommendation for removal of pond Starwort is mechanical aka handremoval. However, there are aquacides that are approved for use in waterways that may be applicable.  You would need to check whatever aquacide label you purchase to make sure it works on Pond Starwort and that it is safe to use with fish/wildlife and other pond plants if you have those.  Check with online pond supply stores or local stores that sell pond supplies.

Aquacide Pellets with 2,4-D are a selective, root killing, systemic herbicide that work on submersed, emersed and floating weeds.  They are for use in ponds, lakes, reservoirs, and other slow moving waterways (not for use in irrigation canals).  They do take a bit to achieve full weed kill which occurs in 3 to 5 weeks.  Again make sure to read the herbicide label very carefully to make sure it will be safe to use in your situation and only apply at the rate recommended for your pond size.