Calendar Events Downloads - issue Google indexing

[Adlago]

Google bot indexing calendar - events - downloads with a link type

http://yoursait/calendar/event/27-name event/?do=download&csrfKey=8ab1eaa1a404d84d83464066fa2e7b8d

But this csrf key is only valid for one session of this google bot. Every Internet user following this link receiving error page.

 Sorry, there is a problem
The CSRF protection key did not match. This may indicate a plugin or theme is out of date. Please contact technical support for more information.

Error code: 2S119/1 

What should be added in the robots.txt to stop indexing event download?