Assuming that the old pages no longer exist (will cause 404 errors), you can do the following: you redirect all pages that no longer exist to the start page. (As indicated in the comments below.)
This is an updated .htaccess code that you can use to make this happen. The first RewriteCond checks if the requested path is a file, the second checks if it is a directory. After that, you are redirected to the start page or to any other page.
http://www.example.com/i/am/an/old/page.html or http://www.example.com/i/am/a/different/old/page.html will be redirected to http://www.example.com/
RewriteEngine on RewriteBase / RewriteCond %{REQUEST_FILENAME} !-f RewriteCond %{REQUEST_FILENAME} !-d RewriteRule . / [R=301,L]
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