Phormium

pampas grass is really hard to get rid of. alot of digging i think. i remember a few years ago my dad tried to burn one out much to the annoyance of the local fire brigade. it still grew back. i would never reccomend that course of action by the way especially close to the house.

just to put the opposite case - if well supervised, (buckets of water or hose on standby just in case!) i had a fab time burning our inherited pampas grass to death. Luckily it was not near our house, but i had to keep a sharp eye on the hedge behind it.
Can't advise on phormium tho.
Good luck!

the fire my dad had was under control. one of his neighbours had called the fire brigade not realising he was with the fire lol. i think op may be too close to the house to burn it out though.

On the phornium. It depends on what you want to do in the soil after its gone,
If you are not using it for six months, you could try cutting it down as far as possible then put a killer on it such as Sodium chlorate this will in effect kill anything thats growing or planted there for about 6 months.
Otherwise just hack it down as far as possible then dig out the roots.