How would you approach this?

We have a client who we have a fixed agreement with - services and labour for any issues. In our contract, which they didn't want to sign, we do have a clause that we will bill for any third party "tampering" as we've called it.

We have quoted these clients for two services, and both services they've decided to purchase elsewhere. They are however ongoing services and we fear that they may need assistance with them at some stage. However, as we have not supplied them we consider it "tampering" and don't want to assist them at no additional cost to them, for any issues that may arise from services not purchased through us. And it's not like we've been unreasonable with pricing, we also know they keep going to our direct competitor after we quote them.

These clients are forever threatening to cancel to get their way, they're bullies really but they're also good payers.

How would you approach this in a way that they understand?