False advertising claims by competitors

[transition]

I'm dealing with an issue with competitors who are trying to negate our advertising claim of having X amount of products. The competitors are claiming they have the same amount of products which is a flat-out lie. The competitors in reality have only a fraction of products they claim to have (proof as generated both by sitemap files and indexed product pages).

Is there any effective way to prevent these type of claims? They're doing this in their Google PPC ads...

[LifeinAsia]

If it's in the U.S., you can probably file a complaint with the FTC. (Other countries have similar agencies.) There's no guarantee that they will act on it.

[MrHard]

You don't have access to their warehouse so how could you prove otherwise? It's possible to have products, but not have them listed online. Or, if a product comes in pieces then there are two there. Two products equals on item.