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 a 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.