If you have purchased a premium key with your host in order to use BoldGrid, then you will need to contact your host for cancellation of the subscription. Additionally, they would be able to help with any refunds if they apply.
If you installed and are only using the FREE version of BoldGrid, then you simply have to de-activate and then delete the plugins from your WordPress installation.
This is not the correct or relevant answer. New users can register through BoldGrid.com and get a Premium Key by upgrading within BoldGrid Central. There is absolutely no way to request for refund there. Your product’s landing page is misleading too. You trick us into believing that the product also provides hosting services which is not true.