Can I refuse to pay for internet service if the speed isn't what I'm paying for?

I have a 100 Mbps internet plan with my provider, but I've been measuring for three months and I barely get 30 Mbps. I've called technical support several times and they say it's because of the area, but I'm paying for the full plan. Do I have the right to refuse payment or demand they lower the price until they fix this?