When running a frequency study the best you can do is roughly restrict the search domain as Siavash Khajehhasani notes. You'd have to have advance knowledge of how the part behaved, though. It doesn't seem like there are many cases where this exclusion would be necessary. For relatively simple parts solving a few extra modes shouldn't take too long.
Once you run the frequency study and you'll understand the shape of each vibration mode. When using these results for any of the linear dynamic studies you can exclude frequency modes with shapes you're not interested in by choosing "selected modes" under "mode selection" in the "results options".
Use caution, though. When I last tried using this functionality two years ago the filter wasn't working correctly. SW assigned a SPR and labeled it "critical" but the SPR is still active.
Well, you could change the boundary conditions so that the torsional modes are suppressed say by using symmetry along the longitudinal axis. Of course you run the risk of suppressing any unsymmetrical modes so it would depend on what you bar looks like.