If players (or player characters) are chronically greedy then the consequences of that greed and having excessive wealth should be felt by them. Remind them in increasingly bad ways that wealth has its downsides. When you have a big wallet people are always trying to find ways to empty it, both illegally and legally.
Strangers, friends, family, business acquaintances, government officials, and even mythical creatures and supernatural beings will all try and separate the PC from their money in many interesting ways including:
- a child begging for a penny in the street
- theft in the street by a cut-purse
- prostitutes drugging you and taking your wallet
- priests insisting on your tithe to save your soul
- a charity asking for donations (especially effective if the PC is GOOD)
- merchants overcharging you because you can afford more
- business proposals requiring investments
- business partners embezzling funds
- friends who want you to pick up the tab
- girlfriends/boyfriends who want you to buy them gifts
- wives/husbands who want more stuff
- daughters/sons who want everything
- premarital dalliances with illegitimate children
- lawyers representing your spouse(s) in divorce suits
- legitimate children who want their inheritance now
- the out-of-control expenses of running your own home/estate
- having your treasury vault burgled while you are away on an adventure
- nobles above you "asking" you to join (and fund) their campaigns against neighboring countries.
- running your own campaign to become leader of your own country
- defending your country from attacks by neighbors
- defending your country from rebels trying to overthrow you
- a dragon taking your gold and destroying your home
- a demigod demanding tribute for all the success they have helped you with
- and of course the ubiquitous tax collectors at every step of the way
This is list is by no means complete, but it should give you some ideas. When the player finally says "Why is everyone always taking my gold?" your mission will be accomplished. :)