1) Put the players in charge of something really expensive, like a starship or a colony. This has the added fun of forming a conflict between their personal purchases and the needs of the expensive project they are leading. "Sure, this one million credits is big money for me, but just a snack for Beowulf.."
2) Decadent player characters. They can easily spend zillions in prostitutes, drugs, parties and other luxury items. My current Traveller campaign is centered around Bowman. The characters are new Baron of the system and his staff. The Baron is very good at spending money in funny things, hehe.
3) Trouble with the law. Having your bank accounts and credit cards frozen can cause immensive trouble for people who are not prepared for a such event. (Mostly concerns planetsiders, travellers need a way to carry wealth independently of banking systems.)
4) Being middle of nowhere. It is always fun to have the players literally swimming in money but with no way to use it.... on way back to civilization, there can be plenty of obstacles that each devour a part.
As a a general rule though, I hate railroading. I throw only "realistic" or at least belivieable obstacles on my players. If they can get through them without losing their loot - by all means, they have earned it.