One of the least ergonomic behavior of Dynamics CRM is this '+' button on subgrids which can take up to three clicks to open a new record form:
But there is a logic behind it, when you go to the related entity menu there are these two buttons "Add new" and "Add existing", which is the default action:
So, the easiest solution is to use RibbonWorkbench (or any other ribbon editor) and to hide this "Add existing" button in the subgrid ribbon of the concerned entity.
So, first you need to dowload the RibbonWorkbench (you can find it here: http://www.develop1.net/public/page/Ribbon-Workbench-for-Dynamics-CRM-2011.aspx) and import it into your CRM organisation.
To open RibbonWorkbench, navigate to the 'Solution'section and click this button:
In ribbon workbench, select the solution containing the concerned entity (solution has to be unmanaged).
You finally need to select this entity (1), in the subgrid section (2) look for the 'Add existing button' (3), right click and disable it :
Click 'Publish' and you're done!
The 'Add existing' button is gone and now when you click the '+' button, a creation form will be displayed.