E.g. if you work at the fed you buy a 3x treasury index fund if you know rates will be lowered, or short 3x treasury index if you know rates will be raised.
If you are a judge or legislator and you get to say if 30% charge on app stores is reasonable, this affects 20% of the Nasdaq 100 index. So you buy/sell that over a 24 hour period.
Disallow them from being directly involved in investment decisions. Make them hand their money over to someone who will make investments on their behalf with influence limited to broad things like risk profile and the term they want returns over.
Alternatively, implement a minimum holding period. It's much harder to trade on insider info if there must be a long period of time from purchase to sale. One place where I worked wouldn't let us sell until at least 3 months after we purchased any shares.