If you're using Professional, this VB script will prevent the New Customer button from doing anything if the user isn't allowed to add new customers.

Imports Sage.MMS.SAA.Client
Imports System
Module addInModule
'Sage.MMS.SOP.EnterNewQuotationForm.vbs
'Sage.MMS.SOP.EnterNewProFormaForm.vbs
'Hide New Customer button if no permission
Public Sub Main
	dim allowNew as Boolean
	dim targetGUID="921e8818-c932-4b6f-8d6f-6148a3dbabc3" 'Sage.MMS.SalesLedger.EnterNewCustomerAccountForm
	SAADesktopAPI.HasUserAccessToTarget(Sage.Accounting.Application.ActiveUserNumber,targetGUID,allowNew)
	if not allowNew then
		form.FindControlByName("accountSelectionGroupBox").UnderlyingControl.Controls.Remove(form.FindControlByName("newCustomerButton").UnderlyingControl)
	end if	
End Sub
End Module

Get your Business Partner to package and install it.

Any ideas if this is going to be fixed in the near future as it causing major credit control issues with a new company that has just starting using Sage 200

Absolutely! It defeats the whole purpose of a credit control system if any user can open a New account and extend unlimited credit simply by entering a new quotation or proforma.