Willows are easily controlled with a variety of herbicides and treatment methods, some of which have already been covered.
One of the aquatic glyphosate formulations (RODEO, AQUAMASTER, etc) should do a very good job on willows. Also, a new aquatic herbicide called RENOVATE is also very effective. 2,4-D is also a viable option.
In all cases, be certain that the product's water-use restrictions don't conflict with any water-uses that your pond provides (drinking, irrigation, livestock, etc.). Also, when conduction foliar (leaf-applied) applications, the addition of a surfactant (wetting agent) to the herbicide:water mixture is critical.
