You, as the tipping administrator, can enter the tips any time.
That means that you can decide if you will accept changes to tips for the later games.
The drawback though is you will have to manually enter the tips, being careful not to change the tip on the games that have already been played.
In other words, I would not be using the post tips direct from email server function as this will post the tips for all of the games (and the tipper may have changed their original tips for the earlier game <aka cheated>.
Personally, I do not accept any changes to tips after the first game has started. It get way to complicated otherwise.