Do they receive some minus to it, or perhaps the inability for some reason? Our Salamander Librarian is rather fond if it, as opposed to soaking damage like a real non-***** Space Marine, and our GM, Commisar McBallin, is seeking some reason to disable or limit the Libbys Dodge ability. Said Terminator Librarian has purchased Leap Up as a Advanced Talent, along with Wall of Steel, so you could see the GM's frustration due to his extra Reactions.

Assuming your Librarian is actually wearing his Terminator Armour....

From page 164 of the Deathwatch core rulebook:

The massive frame of a Terminator is not without limitations. A Terminator cannot Run, and the wearer suffers –20 to Agility, and cannot Dodge (but may still Parry).

Goddamnit, I read that page, musta looked right past it. Thx Prince.