Saying "no" to early paying users without causing them to churn

When a customer who pays your bills asks for a custom feature, saying no feels almost impossible. But we learned the hard way that building every single request turned our app into a bloated mess of half-baked tools. Most requests are just users trying to patch over a workflow problem they don't know how to explain yet. How do you tell high-value clients that their requested feature isn't on the roadmap without completely ruining the relationship?