If you introduce a land value tax equal to the value of the unimproved land, this will kill speculation, but also make it instantly impactful to the citizens that there is not enough supply. This will mean that the people able to change the zoning of the city will instantly see a reason to change the zoning, as better zoning means more supply means lower taxes.
We don't need no speculation.
For example, a speculator moves into a nice small village, sees potential and brings in a lot of foreign capital in a short amount of time. Buying up houses and outbidding potential new citizens. The price of land is also driven up from this, even if it's only a short term thing.
The damage has been done there and it had little to do with the number of houses. It was just a hostile takeover of a community.
/s
When the speculators run out of bigger fools the bubble will pop.
Crypto can at least just be purely speculative without really hurting anybody but it's not good for housing.
Cracking down on speculators won't help people get roofs over their heads. Nor will calling them unethical. Speculators are the signal for whether there is going to be a housing problem in a few years time.
We should want a system that allows speculators and encourages them if there is going to be a housing problem so there is plenty of warning. Targeting speculators in particular is the old once-a-metric-becomes-the-target-it-becomes-a-bad-metric problem in a new guise.