I like Stereo's dpc under the fencing idea, must remember that one, as when I strim each year it makes holes in the chicken wire.
We used to use sodium chlorate when it was allowed and then on to Roundup - but to be honest I think the best method is just to do as someone has already said - a flame gun - you can buy big ones you attached to a large gas can on a wheely thing or you can get aerosol size ones - does it quickly - maybe Roundup after that to keep the newbies from coming through for a while.
Trouble is with gravel the weed seeds make a nice wee bed in between the gravely stones so a dpc underneath gravel has limited use, they have to be killed from the top side too and in rainy summers like this one, it's hard to find enough consecutive dry time to let the Roundup have time to burn in.
Tarmac, there's your answer