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”
Shameless Null Neutrality
Below link has some additional information on the Custome message..
http://www.mmcug.org/blogs/Lists/Posts/Post.aspx?List=41d6c66e%2D7ea5%2D471b%2D89f4%2Dc83b5185f1ba&ID=19
Hope it helps 🙂