Start now! Dig up what you can and regularly cut off all new growth that you can't dig up! You will need to be vigilant. You can sometimes limit the spread by putting a root barrier round the plant but it might be too late for this.
It's a difficult problem. Good luck!

Someone gave me a "Dwarf Bamboo" which I planted in my border. It spread like wild fire. I hacked it down, it came up again. I pulled out as much as I could and it still spread I resorted to spraying it with weed killer and it still came up, sometimes yards away. It took me years to get rid of it completely... Never again.

My advice is never grow ANY bamboo in the ground unless you have a large forest. If you haven't then keep it in a pot.

The only sure way is to dig a trench two foot deep around it. Then dig out what you can and be vigilant when it pops up again, keep the trench until it has all gone (no sign for a year!) It will root out at the edges, but should not be able to bridge the gap.
It is a sad thing about bamboos - and one we ALL learn the hard way - they are fine in a pot, they are fine if a clump-forming species planted out. BUT, if disturbed - as soon as you move them, or divide them, they will suddenly become runners!
I had a wonderful massive stand for 12 years, then I let someone dig some of it out - and it stated to run (and hasn't stopped! I dug a trench the side it was a problem and that has stopped it.
They are contrary plants...

Agree with all the above! Thugs, they are. Dealt with severely they should be. Grub them out as soon as they appear. Rip up the roots that run.

I would only plant them in a contained enviroment. In open ground limit them to a shielded area - ie, dig a trench at least two feet deep and line it with slate tiles or something similar.

In a large pot when they reach the tipping point; cut them in half and discard or pass-on the remainder.

I inherited two of the blighters. I do like them and they remain; Though the thuggish one is planted to hide the oil-tank, it is worst of the duo. I have a constant battle with the brute. I will (eventually) get rid. I would rather the screen of the tank be removed than let it take over.

The other is not so bad being in a far corner. Even so, I am aware of its prescence as it throws up runners every now and then. A sharp spade close to the parent keeps it in trim. But age takes its toll and I think that perhaps the end is nigh for both!

Cheyenne, this is obviously one of the bamboos that take over, ie a thug, I would personally put it back in a pot, larger than the one it was in. If you want to grow bamboos in borders then google which varieties are best, I have black ones (nigra) in my borders.

@booboosmum Phyllostachys nigra are also a running bamboo. They behave themselves until they settle and then make a run for it: to forewarn you.

Fargesia are the only bamboo for me that do not run, as they are clumping.

I agree with Kildermorie I have a couple of the clump-forming Fargesias. I still keep them in planters though, as the thought of having to deal with them if they spread makes me a bit wary.

I love Kildermorie's comment "They behave themselves until they settle and then make a run for it" - makes them sound like Ronnie Biggs!

I've heard that pig troughs make good bamboo pots - shallow, narrow and long means that you can get a screen rather than a clump and the roots can't get through the iron.

Kilder, yes they should behave in the 1st 10 yrs in uk climate, I bought them cos Alan Titchmarsh said they weren't invasive!!!

I had one in my front garden, never again! My advice is don't ever plant one directly in your garden as it will spread absolutely everywhere and you'll never get rid of it. If you are desperate for a bamboo then keep it in a pot.

The ticket on mine also said Alan Titchmarsh "recommended" for small gardens....mine are pushing out the pots and are not happy but they are NOT coming out (:0)