How do I make all players pay online in advance?

Open Settings, “Finance settings”, “Accepted Payment Methods” and choose “Only Online”. Then players in all your games pay online when they join (card, Apple Pay, Google Pay or Playbox wallet), cash is not offered, and the money lands on your Playbox balance after the game. There is no per-game switch.

For one problem player only, keep cash for the rest: open his profile, tap “Ban or restrict” and choose “Online payment only”.