There is no “money” in FiveM by default so whatever is allowing them to influence your “economy” should be fixed. You’ve got some vulnerable events
Keep track server side what weapons each player is allowed to have and periodically (your own discretion, maybe 10 seconds) check if they have weapons they are not allowed to have. If so, ban/kick/do whatever.
See How To Create an Anti-Cheat - List of Vulnerable and Abused Events (Updated January 2020) for a list of known vulnerable/exploitable events and how to combat them.
Another helpful post How hackers can exploit your servers and what to do about it
Good starting point for an anti-cheat for 
servers [Release] AntiCheese Anticheat
TLDR: You have to make your own anticheat suitable for your server. Check abnormal behavior.