Step 1.
arcgis server admin directory is NON-graphic, text only interface to maintain arcgis server. 
arcgis server manger is user-graphic-interface, does the same thing

 
 
 
Step 2.

 
 
 
Step 3.
you can delete REST service here

 
 
 
Step 4.
 
 
 
 
 
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Step 20.