Many people believe "sanctions" are just weak empty threats, they are not. It is a general term for an almost unlimited list of actions that could be taken to literally ruin Russia. It may not change Putin's course but it can burn down his country around him. Sanctions can starve them out financially, technically and even literally. It will however take time for them to work. We are talking months and years, not weeks.
However, because Russia is the number 2 producer of oil in the world, the west will also feel the pain if they stop buying that oil which is Russias source of income. There is certainly more to the subject but you can see why the sanctions such as removing them from the banking system which would keep them from getting paid for their oil (stopping sales) needs to be implemented AFTER alternate oil sources or plans are in place. That of course is a big deal for EU especially since they use a lot of Russian oil.