Every limit is a commanded outage. We can refund an unexpectedly high bill. We can't refund downtime.
So write "WARNING: your service will go down if you exceed this limit. We cannot provide refunds for this. To avoid outages in the event of unanticipated traffic, do not enter a spend limit."
You’re optimizing for one class of customer by denying another class a choice they explicitly want.
So write "WARNING: your service will go down if you exceed this limit. We cannot provide refunds for this. To avoid outages in the event of unanticipated traffic, do not enter a spend limit."