This is a nightmare weed to remove. You have to dig very deep and get every last piece of root out otherwise it will just regrow. One option I've read about is to get a systemic weedkiller spray, put each individual piece of weed above the soil into a polythene bag and spray it, so that gradually the weedkiller is ingested, travelling down to the root and killing it. But this is a time consuming activity and you can't just spray without protecting other plants otherwise they could all be killed. If you can afford to, leave one patch of soil clear of plants at a time, say for one season, and just keep digging away at it until every last bit has been removed.