How to slow them down so they can think more thoroughly?
First, they themselves should realize such problems exist. If not, they will not listen to any suggestions.
Second, use some mechanical routines to slow them down. For example:
- Sit on their hands all the time, only release their hands after they finish thinking and decide what to move.
- Write down moves first before making the moves on board. (against FIDE and USCF rule, but USCF tournaments silently allow the practice)
- Write down time used for each move, or at least the moves that take long time. (against FIDE rule, see Wesley So incident in US Championship)
Third, play more in long time-control tournaments, then slowly change their habits.
Fourth, remind them there is no need to rush in long time-control tournaments. Show them examples with your own actions:
- Do not rush to find pairings. It's always crowded when the pairings have just been announced (posted on wall).
- Do not hurry to finish eating food during breaks, even if the round has already started. (FIDE has zero tolerance rule. USCF never has it, 30 to 60 minutes grace time is normal)
- Do not run to restroom.
- Do not move right away after returning from restroom!!! (sit on hands and wait or count the clock for 3 minutes first!)