I have a technical peer at a customer who I work with several times a week. I handle high-level support requests from her. Typically both of our PMs are CC'd so they can be in the loop as we address issues.
Frequently those PMs will hijack a technical support email thread with unrelated questions.
How can I politely discourage this behaviour? It makes it hard for the PMs to get clear answers and it makes it hard for me to continue providing technical support.