If it is between plants you can dig up the good plants, then the bad stuff. Yes it is a pain!!! I have some really nasty, spready perennial that the roots I swear go down a mile!!! So with those I dump a bit of full strength round-up in a small glass container and use either a q-tip, small craft brush or small foam brush with a bit of the round up and dab it on the bad plants' leaves. It takes a bit of time but sometimes that is all you can do.
Icky spready perennials never to plant: "Ladybells", saponaria, "snow, on the mountain", lily of the valley, garlic chives!! Ladybells for instance (adenophora) has shot roots attached to really deep spikey roots. I have to dig about a foot down. Rototilling makes it worse!! So I battle that plant yearly!! Some people like creeping phlox so dig it up and give it away!!