You can run a quick Apex script on either UserLogin or PermissionSetAssignment.
WARNING: If you modify these scripts, be careful not to lock out your own login. Support would be required to fix the problem.
Example #1
UserLogin[] logins = [SELECT UserId FROM UserLogin WHERE UserId <> :UserInfo.getUserId()];
for(UserLogin login: logins) {
login.IsFrozen = true;
}
update logins;
Example #2
PermissionSet lockoutPerms = [SELECT Id FROM PermissionSet WHERE Name = 'LockOutUsers'];
PermissionSetAssignment[] assignments = new PermissionSetAssignment[0];
for(User userRecord: [SELECT Id FROM User WHERE IsActive = true AND Id <> :UserInfo.getUserId()]) {
assigments.add(
new PermissionSetAssignment(
AssigneeId = userRecord.Id,
PermissionSetId = lockoutPerms.Id
)
);
}
insert assignments;
You can use the Apex Data Loader as well, or any other API-enabled tool to create/modify these records. To reverse this when you're done, simply update the UserLogin records, or delete the PermissionSetAssignment records.