To answer your first question, "Is there a way to turn off the internal IR signals coming from the Slingbox 500 unit itself?", no there is no way to shut this feature off.
And to your second question, "Does anyone know how to use a standard IR EMITTER cable with a Slingbox?", keep in mind that if a different External IR that was not supplied by Sling is used, this would not be supported. If anything happens where the remote stops working, an agent would recommend to use the Sling Media External IR.
Hope this helps!!!
Sling Media Moderation Team