Please find the below commands to set Custom Quota Messages for Warning, ProhibitSendMailbox and ProhibitSendReceiveMailBox, Just change the require custom message and apply the same.
New-SystemMessage -QuotaMessageType WarningMailbox -Language EN -Text “Your Custom Message”
New-SystemMessage -QuotaMessageType ProhibitSendMailbox -Language EN -Text “Your Custom Message”
New-SystemMessage -QuotaMessageType ProhibitSendReceiveMailBox -Language EN -Text “Your Custom Message”