Is there a way to disable the wysiwyg editor when editing mail templates?
Editor can be disabled completely by the following 

Administer > Customize Data and Screens > Display Preferences.
CKEditor  can be changed to TextArea

http://book.civicrm.org/user/current/initial-set-up/customizing-the-user-interface/



For specify Action 

Editor can be disabled before editing the template using SQL 

civicrm_setting table holds the CKEditor setting 

Id: -         11

Group name: - CiviCRM Preferences

Name: -        editor_id    

Value: - s:1:"2";

If the value is made null then it changes to Text Area and later it can be changed back to CKEditor 

But, any member accessing the Editor at the every moment like in contact summary screen for creating PDF at the same time cannot view Editor till this is been changed -