Govt Can Make Illegal Anything Within Their Region.
If So, You Wouldn't Be Able To Legally Transact Using Bitcoins Without Some Penalty If You Get Caught.
Perhaps There Are Dark Markets Where You Wouldn't Be Caught, But This Would Severely Limit Bitcoin Adoption Within That Country.
Although There Is Nothing That Can Stop A Govt Or Central Bank From Banning Bitcoin,
Most Would Not Go That Far For Two Reasons:
Banning Bitcoin Is Too Difficult To Enforce Or
The Government Does Not Know What To Do With Bitcoin