Most, if not all governments, have some form of sales tax. What happens to bitcoin value when governments prohibit use of bitcoins to curb evasion of taxes
your premise that governments will ban bitcoin to curb evasion of taxes is invalid, but I will attempt to answer your underlying question:
"What happens to bitcoin value when governments prohibit use?"
Just regarding value (compared to State currencies):
The value drops based on investors getting scared and mass selling.
The value rises as fiat currencies continue to inflate and the need for Bitcoin continues to exist.
Governments can't prevent people from using Bitcoin. They can make laws, but it doesn't affect the need for stateless forms of exchange.