To delete contact, login to the MSPControl and click on Contacts,
or go to Organizations > Exchange > Contacts.
You will see the list of all existing contacts. If you have many, it might be a good idea to use Search form in the right to find the one you need. You can choose whether you want to search by Display Name or E-mail Address.
Once you found the contact you were looking for, click delete icon in the right.
Click OK when prompted.