So has the title say I want to know if it's possible to overwrite the entire internal hard disk with Ubuntu? I've messed up with windows partition (uff) and that won't boot so I'm asking this to currently boot only ubuntu
It sounds like for your purposes it's enough to simply format the drive, not overwrite it.
Overwriting would be necessary if you wanted to make sure no previously saved data would be recoverable in the future.
But it sounds like you don't need that. Rather, you need to format it, and perform a clean install of Ubuntu.
Yes, this is totally possible. Boot from an Ubuntu Live DVD or Live USB. Go through the steps to install Ubuntu. When you see this step, select the top option like so:
Please note that by doing this, you will make it impossible or very difficult to retrieve any of the data (personal files, etc.) currently stored on this HDD. But you say in OP that this is what you want.
Depending on how hosed up your Windows installation is, you may also see something like this. Regardless, the choice is the same.