Step 1 is to put the scope of each project in writing before the work begins, clearly enough that you can refer back to it when something falls outside it. Not as a legal document, but as a shared understanding: this is what we do and this is what we do not do.
Step 2 is to address it the first time something falls outside the scope. Not defensively and not apologetically. Simply say: this falls outside what we agreed on, so let us look at how we can resolve it. That conversation feels uncomfortable the first time. Every time after that becomes easier.
Step 3 is to stop looking back at what you previously gave away. The exception you made last month is over. The boundary is in place now. Anyone who refers to what was previously allowed is told that the standard has been tightened.
Growth sometimes means pruning. That applies to clients who are not a good fit and it also applies to working agreements that have become too broad. Agency standards describe the level below which you refuse to fall. Good Growth vs Bad Growth shows what happens to your margin when you do not protect that level. And the H in SHARP is exactly about this: holding your position when things get busy.
Which client received something outside the agreed scope during the past month, and what would you do differently now?