Cut it back to the ground Suzymorgan and then dig out the roots. It will be a challenging job but worth it in the end. Have you any ideas as to how you will plant up?

Or use a weedkiller on them, although you will still have to dig them out later. They aren't too bad as long as they aren't growing out of cracks or in other awkward places. Maybe use weedkiller on these ones. You may need some kind of security like a fence or gates to stop more dumping. Try and get other people involved too. How about a 'dig and BBQ day'?

I've spent years having to deal with brambles - cut them down as far as possible, loosen the soil around the base of each one -if you find a good sized knobbly, woody lump, leave it in place. Now take a drill with a medium wood bit on it, drill into it to make tunnels about a quarter to half inch deep (don't drill straight through) and fill the tunnels you make with SBK (brushwood killer from the garden centre). Try not to spill it on the soil, its very watery, and wear rubber gloves. Now cover the holes with something - I usually use upturned flower pots of a size suitable to cover. Now, if you want, and you've made a deep hole, replace the soil around and on top of the pot. This serves two purposes - the pot keeps soil away from the treated root and makes it much easier for you to find again if you need to re-treat, plus stops you inadvertently planting where you've applied SBK. As for the rest, if you can dig out those with smaller roots, do.