It may not be easily seizable, and with the right security practices it may be nearly impossible to seize directly.
But governments could announce certain coins are not spendable and enforce that on companies or people subject to their jurisdiction. Prosecuting anyone who accepted one of those coins with a crime.
Which is why privacy technology is being implemented in blockchains like ZCash, Monero and Ethereum.
Those types of tyrannical governments already have people operating in black markets. Crypto doesn't really change that it just makes the bad government's job harder which the vast majority of the world would agree is a good thing. The people of Sudan, Syria, Turkey, and the Phillipines among others are grateful that cryptocurrencies give them an option of having freedom.