There is a product, used in organic agriculture to control slugs and snails, that could be efficient to control vermetides: Ferric phosphate.
The trademark of the product is Ferramol® Organic, but for the purpose of experimentation I believe that the use of saturated GFO may be attempted by the aquarist. Ideally, the saturated GFO should be dry and ground by dosing the product on the display when the vermetides are with their nets extended to ensure they can ingest it, and then wait for the results.
There is no defined dose to use for this purpose. Caution is required.
The mechanism of action is explained here:
The potential for slug control with ferric phosphate
It seems to be safe for animals except slugs and snails.
Best regards