First of all, if the loads are defined within a STAAD.Pro file, it is not possible to exclude these from the analysis unless you do one of the two
a. Comment out these cases from the input by putting an asterisk mark in front of them in the input command file.
b. Define these as reference load cases
However even if the structure is analyzed for all cases, you can always filter the post-processing results for certain cases and for that you do NOT have to define envelopes. All you need to do is to go to the post processing mode and simply right click your mouse on any of the tables and choose Results Setup ( can also be invoked through the menu option Results > Select load Case ). Subsequently can you bring the appropriate cases to the Selected window and click on Ok and then the results will be filtered based these selected cases.
The above filtering can also be done based on envelopes. One can add LOAD ENVELOPE by going to General > Load & Definition > Load Envelopes. One can select the Load Envelope from the tree structure as shown next and add an envelope consisting of appropriate cases.
Subsequently the filtering can be done based on envelopes as per the same procedure described above.
Defining envelopes has some other advantages too in terms of design. For more details on Envelope definition, you may refer to Technical Reference section 5.40 titled Load Envelope.