If you wish to hide your trades from a contact of yours you can block them or remove them from your followers list.
Detailed instructions for removing a user from your followers list or blocking a user can be found in the Managing connections on Shares article.
Alternatively, you can set your trade settings to private, which will hide the trade from other users on Shares. No one will see the trade and will be aware that you made it.
You can do this by changing the trade visibility settings in the app when placing a buy or sell order. Please see this article for more information on how to do so: How to define the visibility of your trades in the Social feed and in your user profile?