@slth:
In case you are wondering how on Windows:
cmd -> diskpart -> list disk -> select disk #yourflashdrive -> clean
by GUI
Control Panel->Administrative Tools->Computer Management->Disk Management..
find the USB stick drive listed, usually by the bottom of the list. Click on existing partition, right mouse click and select "delete.." from menu.. then create new partition by right clicking on raw drive volume and selecting "New Simple Volume"..