There are many reasons someone would want to edit their post, I don't think editing a post should automatically disqualify it. Making your internal records of who did what (with quotes or whatever) is more reliable.
It has been suggested that the blockchain could be used as a timestamping service for arbitrary data, people could timestamp a hash of their play. But the infrastructure for that is not yet fully developed.
On the default theme, if they edit their post after a few minutes have passed since it was posted, a dotted line will appear under the time of their post that can be hovered over for more information. I'll edit this post in a few minutes to show you.
Edit: Now edited.
Wow, I didn't know this.