How to hide grower class from main categories block

[texscalper]

Hello,
I'd like to cancel the grower class (the plus icon ) from the top main categories in the tree.
I'm using the a modified version of the category block (the optimized one).

I want to add little symbol images only for the main categories (one for each category) and make these symbols work like the plus icon (grower class).

The grower on subcategories will behave in the same way.

I've tried to see into the category.php and .tpl and something in tools\treeManagement.js but could not work it out.

Any help would be appreciated.

Thanks

Claudio