I don't "warn". If they drop below 3 stars, I delete...period. The only exception are those that have mentioned they were going to be away and/or long time neighbors that are taking breathers from the game. Even then, though, I'll give them 2-3 weeks, then they're gone. As someone else said in here, it's a game. I don't hold any type if emotional attachment to other players.
Farm Story 2.0.2
Device: iPhone 4 (soon to be 5S!)
iOS: 7.0.2 (11A501)
Instead of just mastering crops, play wth farm design. go to the thread showing pictures of farm design. very challenging, less nbrs, better friends, I delete nbrs at 2 wth3 reminders, just a thought....