Milk spilled in warm parve soup in meaty plate, is the plate still kosher ?

Shalom, Thank you for your question. If the heat of the soup (and the milk) was less than "yad soledet bo" (about 40-45 degrees – to hot to hold your finger in), then you just need to clean the plate out. You should do so with water that is also only lukewarm, or colder, so as not to heat up the forbidden milk in the meat plate. Blessings.