Changing how that works is not something supported by the theme. You can add custom code to make a change like that though. You could take out that whole post format selector part at the top all together, and I think that would solve the problem for you. To do it, you will need to get your hands dirty and dig into the code, but it sounds like you're willing to give it a go!
1. First, you will need to create a child theme. You should follow the instructions at http://codex.wordpress.org/Child_Themes first and get a child theme copy working and tested before continuing.
2. Once you have a child theme setup, copy the post-format.php file from the original theme to your child theme folder.
3. Open the copied post-format.php file from your child theme and remove lines 31 to 36 — which are the ones starting with
<ul id="post-types"> and ending with
4. Save changes and test adding a new post to make sure everything works properly.
Note that these instructions are based on version 1.5.4 of the P2 theme and may not work for other versions.
What the change effectively does is remove the option to set any post format on the front page prompt box and that way the posts will always default to the standard post format. It's a bit of a hack, but I think it will work for your use case.