For fonts i wouldn't have the answer...but for brushes here is what i did before:
go to c:/program files/gimp/share/gimp/2.0/brushes
the path should be the same. Select the various brushes you want to erase (ctrl+ a left click to select all the ones you want) then just press "del" or "suppr" (of your keyboard obviously
) it should theoretically erase the brushes you selected. As i said it's the way i did when i had to erase unuseful brushes i downloaded...hope it would do fine for you..