Trimming down a big SQL Server database
Select the second radio button Reorganize pages before releasing unused space, and then in the Shrink file to: option, put the minimum MB specified. You will be corrected if you put a lower MB than that specified. Then click OK. 

I have found that using the DBCC commands for these work all the time, as the GUI did not work in some of my cases. 

Also note, the extra space is there for a reason. You will need the space for future growth, but then that is a whole new topic in performance.