Threaten to file a complaint to the FTC, as previously mentioned.

In general there is always someone you can complain to.

Also often times there are local mom&pop places that are resellers and pay to piggyback on the line. Except they tend to actually provide the advertised speeds. Only problem is they also tend to be a bit more expensive. Although if you only have the one provider, I don't know if the resellers are even a thing...