hi dhamps,
my 2 cents....
You can create a volume in Pro/mech under
Structure --> Feature --> VolumeRegion-->Create
and once u've created the vol. try constraining it in all 6 degrees of freedom. I havent tried it myself yet but see if that works. Also if there is say a "cylinder" you wanna constrain then i would just constrain the two opposite faces to make things simple. hope that helps.

ym

One way you can do that by assigning a value of young's modulus (E) for that volume order or two magnitudes higher than the other materials in your structure. Say if you have structure made of steel (E=29E6 psi), then define the E=10E10 psi for that volume which you do not want to be deformed.

Note different parts of the structure basically act as bunch of springs. By defining artificially higher young's modulus (E) you are making the spring representing that volume as stiffer than the other volumes. This will produce relatively very low (or insignificant) deformation in that volume.

C if it works for U.