Establishing causation: short-selling bans and volatility

Regulators imposed a short-selling ban on some stocks (say, financials) but not others during a crisis. You want the causal effect of the ban on volatility, but you cannot randomize which stocks are banned, and the banned names were likely the already-turbulent ones.

Describe how you would estimate the causal effect from this observational setting. Name at least two identification strategies and the key assumption each relies on.