Access your receiver? If it's one of the DVR models, yes.
Control what commercials you see? No.
You can remotely access your DirecTV DVR and watch TV or pre-recorded shows from it. So, someone else could also access it -- if they knew your passwords and all that.
However, accessing your DVR does not mean they can control the satellite signal, they couldn't influence what commercials would be shown or suddenly force your ESPN channel to switch over to Cartoon Network.
No, it's not even feasible to do this either. It'd just be far, far simpler to simply unplug your receiver from your TV entirely, and just send my signal - whatever I wanted - to your TV directly.