So that leaves us with selling bonds.
As for the IOUs from banks: It is an interesting question how much stocks are bought with that kind of pseudo money. Since banks probably do not give those IOUs to people who want to buy stocks with them, I would think not so much.
The IOUs would have to go through zombie companies which give the IOUs to their employees who then buy stocks with them.
It just has to stop buying them, and maybe actually start selling from their giant portfolio...
That's not a reasonable take (in the context of the question). Controlling reserve rates controls the money supply, and the money supply is relevant when it comes to the level of inflation in the economy. What you're referencing is the 'monetary base'. (which is actually only a relatively small fraction of money in circulation).
I explained why I think for that for this context the monetary base is the right type of money to look at.
For crypto and stock we do the same. People say Bitcoin is capped at 21 million and stocks in circulation is the amount the companies issued. If someone owes someone else a coin or a share, we do not count that towards the supply.
selling bonds or increasing reserve requirements are two of the fairly straight-forward tools central banks have to reduce money supply.