I think the Russians need to do this. Western policies can contribute to nudging them to do this. If the Russians can't or won't. I really don't know how he can be ended except from natural causes. The most likely Russian way is a coup from the military or FSB, but Putin is so paranoid that clearly anyone that he suspects of betrayal gets thrown out a window. That's classic evil dictator behavior and he fits the bill.
I don't remember the source but I recently saw a pundit saying the people COULD oust him if a million of them hit the streets in Moscow. While I don't know if that's true, or perhaps two million or another number would be needed, but up till now Putin's intimidation tactics and machine have always worked. That seems the best way, a real people's revolution, but somehow the least likely.
Putin Has Left the World No Other Option But Regime Change (thedailybeast.com)