Can you tell us exactly which Preen product it was, there are several:
http://www.preen.com/products/...
Edited to add if it was Preen Garden Weed Preventer, that contains trifluralin. Another product that contains trifluralin plus another herbicide says it gives "partial" control of several Euphorbia species. So maybe it isn't the best herbicide for that weed. Your best bet might be to call the toll free number on the product and ask them. Or just keep pulling/hoeing it out until the seeds in the soil are gone. Make sure not to let it produce more seeds!