If you wish to exclude a product from showing up in recommendations universally across your whole site (Perform recommendations) and all emails (Reach recommendations), it is best to do this at the feed level.
If you use the Episerver Personalization RSS feed format, you can set the <p:recommend> element to value N for any products that you do not wish to be recommended.
Please see page 10 of the Catalogue feed specification.
If you wish to exclude a product from showing up in a specific on-site widget or email widget, this can be done through the Personalization dashboard. Please visit the articles below for instructions.