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 ghost mode, this way all your followers would not see your trade details, but will instead see that someone acted anonymously.
You can do that by changing the visibility settings of action, like sales or purchase.
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?