Remove " Sent via ERPNext "

Dear All
We start using ERPNEXT and when we send first quote below quote shoe " Sent via ERPNext "  .
it's very bad if erpnext do marketing like this just loosing big customer and get home user and small customer . never big company accept send advertising for his customer on this way  , any one know how we can remove it ? 
it's our secure what ERP software we using . 
If any one know how we can remove please let me know . 
Regards
Meisam




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