Recognising the limit.
WHEN A CLIENT HAS OUTGROWN YOUR ALLOCATION
Their legitimate usage consistently consumes a disproportionate share Their traffic is genuinely large They need resources a shared allocation cannot provide They need root access or software you cannot install
HOW TO PRESENT IT
As their success, which it is. Their site has grown beyond what the plan supports.
Not as a problem with them.
THE OPTIONS
Their own shared account, if the issue is your allocation rather than shared hosting generally A VPS, if they genuinely need dedicated resources
WHAT YOU CAN STILL PROVIDE
Management, maintenance and support, on hosting they hold directly.
That is frequently a better arrangement: they hold the hosting relationship, you provide the service.
And your margin on management is better than on reselling capacity.
WHAT NOT TO DO
Keep a client whose usage degrades everyone else's service.
Every other client pays for it in performance.
TELLING US
Open a ticket if you need advice on what would suit them.
