Maybe just use the textbox's BeforeUpdate() event to clean up the string?
e.g.
Private Sub txt_BeforeUpdate()
'use the .Text property, not .Value on this function only
txt.Text = replace(txt.Text, vbcrlf, empty)
End Sub
That's an extremely (over?)simplified example of what you want to do. Anyway, you can't disable paste ***properly*** in Access (though you might be able to disable one or two ways to do a paste operation).