I don't bother to 'threaten' or 'warn' - depending on your stand point - advance ticket holders because the terms are made clear on the ticket and at time of purchase (particularly on the new stock) so I take the default approach of if they haven't asked me first with a decent reason I charge them. If I go round and they've been delayed, had ticket acceptance granted or there's a really obvious issue I let them go there and then.
If however I am stood at a station where people ignoring ample ticketing facilities is a problem - Nottingham and Liverpool Lime Street are two of them - then I do make announcements effectively threatening that people who have ignored the signage, ticket machines and ticket office will be liable to pay far more than they thought and to give them fair warning to get off and pay or take a large hit in the wallet area. Lime St in particular is awful - I don't know what the two blokes in the boxes are there for but it's not checking tickets, it seems to be chatting about the football.
If however I am stood at a station where people ignoring ample ticketing facilities is a problem - Nottingham and Liverpool Lime Street are two of them - then I do make announcements effectively threatening that people who have ignored the signage, ticket machines and ticket office will be liable to pay far more than they thought and to give them fair warning to get off and pay or take a large hit in the wallet area. Lime St in particular is awful - I don't know what the two blokes in the boxes are there for but it's not checking tickets, it seems to be chatting about the football.