Delete all users and there content apart from user 1
You can use Views Bulk Operations (VBO) module...

Above module allows to execute bulk operations on nodes, users, comments; it also permits to apply the selected operation to all the nodes, users, or comments using the Batch API. The batch API allows forms processing to be spread out over several page requests, thus ensuring that the processing does not get interrupted because of a PHP timeout, while allowing the user to receive feedback on the progress of the ongoing operations.

There is a setting in the View's VBO options to enable a select all button that select all items across all users and unselect admin.