I’m deploying SPLA licenses on my customer's premises, on their server. How do I ensure that they don’t use our licenses should our relationship with the customer cease to exist?

The key here is for you to include the Microsoft End User License Terms as a part of the Terms and Conditions that you have with your end-customers which cover most of the license terms that the end-user must adhere to. For e.g., see below the clause around “termination”:

 

TERMINATION. Without prejudice to any other rights, Customer may terminate your rights to use the Products if you fail to comply with these terms and conditions. In the event of termination or cancellation of your agreement with Customer or Customer’s agreement with Microsoft under which the Products are licensed, you must stop using and/or accessing the Products, and destroy all copies of the Products and all their component parts within thirty (30) days of the termination of your agreement with Customer.

 The term “customer” from the above excerpt refers to you, as the SPLA partner.

 

Note that End User License Terms is provided to you as an Appendix of your SPLA contract.