They can legally bind the revenue to expenditure purposes when they pass the spending bills. I have actually seen this in a few states and municipalities. I believe NJ passed something within the last year or two legally binding revenue from a specific tax to be spent by a certain legislative authority. If I recall, it was related to the gas tax - they increased it and they said everything must be spent by whatever government entity is responsible for highway maintenance (DOT I think?). That way there is no legal way (without another bill being passed) for politicians to use the money for some other purpose. It's still possible that the DOT squanders the money through mismanagement, but at least it would still be spent on roads and infrastructure, as intended.

Same way that private corporations ensure money is spent where it is intended to be spent.

Fire (read: impeach/do not re-elect) those that do not their job by spending money not for its intended purpose. Discover this by regular, full, public audits.

Prosecute those who commit embezzlement and larceny and send the to prison if convicted. Make them serve long prison stints with no hope of parole for having violated their position of public trust and bar them from holding public office or lobbying positions on penalty of more punitive prison time.

Privatization.

Laws with criminal penalties.

Kinda hate to say it because of the current level of hype making it always sound buzzwordy, but a blockchain or something similar could be legitimately useful here. The issue is getting government entities to agree to strict accountability.