Prepare a CSV File to Import New Exchange Online Users:
http://help.outlook.com/en-us/140/ee441218.aspx#required

You can refer to the steps below to bulk remove the users:
1.Create a CSV file that contains all the users you want to remove:
The CSV file should include the five required attributes: Name, EmailAddress, FirstName, LastName, Password.

 

2.Install and Configure Windows PowerShell
  http://help.outlook.com/en-us/140/cc952756.aspx
3.Connect Windows PowerShell to the Service
  http://help.outlook.com/en-us/140/cc952755.aspx
4.Run the command below:
$users =Import-Csv C:\Users\...\Desktop\Password.csv
$users | ForEach-Object {Remove-Mailbox –Identity $_.EmailAddress}

Note : You should find the path of the CSV file, and use the path instead of C:\Users\...\Desktop\.