Bored Gerbils want to do nothing but gnaw?

I have two boy gerbils, I take good care of them, clean their place, feed them, give them water, stuff to gnaw etc. But if I don't constantly give them card to gnaw on, they try and gnaw the lid of their tank thingy or anything else they can get. I tried giving them climbing things and other toys, but anything I give them to stop them being bored they just lose interest and gnaw it to bits!

Please help me I don't know what else to do stop them gnawing at things i really don't want them to.