I'm having the same problem with purple violets - I didn't realise that's what they were until recently. They've taken over everywhere, including in between the cracks in the block paving. I'm pulling them up wherever I see them, before they can set seed, but it's a ongoing chore. I certainly never planted any violets, and I wouldn't mind if it was the only riotous plant in my garden, but a few others have joined the club now too. I do wonder if the fact that I use organic compost might be responsible for bringing the seeds in?