If the people of a city wanted to request that their mayor be removed from office, what tool of "direct democracy" would they most likely use?

Direct democracy is the type of democracy where citizens exercises their political decisions directly without passing through elected minority.
 a direct tool that can remove a sitting mayor from power is collecting signatures. the number of signatures vary from jurisdiction to another. Another tool would be mass action against the said leader. they can also wait for the scheduled election and exercise their rights in the ballot.