If the bottle is working properly and they are just knocking it off for something to do, you need to attach it better to the side of the cage.
By the way, wood toys for chewing are worthless for guinea pigs. Their chewing teeth are in the back, not the front. They need unlimited good quality hay to chew to keep their teeth healthy.
WICharlie, don't worry, they do have unlimited Timothy hay. I just got them the wood chews because they like pushing them around and they seem to enjoy nibbling them. As long as they aren't unsafe, I just use them to help the pigs' boredom.
Additionally, something that I saw after posting this is about adolescent guinea pigs. Mine are around 8 months, so they are at the age when that could be occurring if what I read was correct. Could that make them more energetic/crazy?