Select VTS_0X_*.VOB, then select the entry in PGCs in selected TitleSet and click Strip. Strip Selection window will pop up and uncheck the unwanted audio streams and click OK. Repeat on other entries is necessary.
As you can see VTS_02_*.VOB has 5 audio streams (#Aud). In the PGCs in selected TitleSet select the Entry that you want to strip and then click Strip button.
The Strip Selection windows will pop up showing all the streams for that Entry. I only want to keep the English 5.1(6 channel) audio stream. Uncheck the other streams and click OK. You will notice the Action column will change form Keep to Strip.