Just a little bit of info about how the theme works in this regard.
The theme respects the default WordPress functionality, which is to list your posts on the front page, unless you select a static front page.
All themes which are available at wordpress.org have to do this or they wouldn’t (or shouldn’t) be allowed on wordpress.org.
Now, you have installed the Blog Designer plugin which as far as I can tell, is used to create and style another blog page in addition to the front page. So, if this plugin is also listing your posts on the extra blog page, then your posts will be displayed on both pages.
As Yazmin mentioned, this extra functionality is provided by the Blog Designer plugin and you would need to refer to it’s documentation or ask the plugin authors how to control the output from this plugin.
Hope this helps.