Well, making it more complicated just so not to use variables <g>

loForm			= Createobject('myForm')
loForm.Show(1)

define class myForm as Form
	add object myTextBox as Textbox

	procedure myTextBox.KeyPress(nKeyCode, nShiftAltCtrl)

		nodefault
		if nKeyCode # 92 or At('\', this.Value) = 0
			Dodefault(nKeyCode, nShiftAltCtrl)				
			if ((nKeyCode = 92) or (nKeyCode = 127 and substr(this.Value, this.SelStart, 1) == '\' and substr(this.Value, this.SelStart - 1, 2) # '\\')) and nShiftAltCtrl = 0
				Dodefault(nKeyCode, nShiftAltCtrl)
			endif
		endif

	endproc

enddefine

[Update] By the way, I realize that you can still press DEL or select text and then overwrite/delete, but do not feel like adding more code, after all this might not even be what Jay needs, maybe I completely misunderstood him.