I apologize for this delayed reply to your gardening question and hope it is still helpful to you.
Roundup doesn't contaminate the soil and in fact you can plant seed in that spot a week or two later and they will grow with no problems. There is not a good product to kill the weed grass but not harm the lawn grass. Your best bet is to build a strong dense turf over time with proper mowing, watering and fertilizing. This way it will choke out most weed seedlings over time. In the meantime continue to do the spot treating of the existing weeds.
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!
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