They could surely try, with varying possible degrees of success.

They could make it illegal for any banks to do any business with Bitcoin exchanges, which might stop large Bitcoin buyers in countries where this was made illegal. This could cause a significant drop in the value of Bitcoin.

However, they could not prevent smaller scale Bitcoin transactions from taking place without implementing a 1984-style police state and completely destroying the concept of free speech.  That isn't going to happen, and if it does, Bitcoin is the least of our problems. (After all, Bitcoin is just people sending messages to each other.)