I'm acting as the liaison between the sponsor organization and the IT department. The sponsor always goes around me directly to other people in the department. How can I get him to work through me as he is supposed to?

First, you've got to avoid the temptation to try to simply enforce policy. When faced with a recalcitrant sponsor, many people in liaison roles will try to use process and procedure as a bludgeon to bring a client into compliance. This rarely works and usually makes things worse, increasing the frequency of the bypassing behavior.

Some go all-out to try to enforce policy, getting others in the department to collude to avoid contact and then reject the sponsor's overtures when avoidance fails. This, too, makes things worse. The sponsor becomes frustrated with both the liaison and the rest of the department.

All of this looks to a sponsor like spoiled children stomping their feet, complaining that they are not "doing things the right way."

So you need to think carefully and dispassionately about why your sponsor is doing this. Of course, that's easier said than done. Being ignored probably feels like a constant slap in the face. It's easy to take it personally and to let it cloud your judgment about the situation. So it may pay to ask a few of your colleagues about their observations of the relationship and why they think this is happening.