You have to customize the xml to obtain the desired behavior. The best approach is creating a new extension from template ybackoffice to consolidate your changes.
Basically, the original extension have these lines:
<context type="Product" component="pcmbackoffice-assortment-collection-browser" >
<cb:collection-browser xmlns:cb="http://www.hybris.com/cockpitng/config/collectionbrowser">
<cb:available-molds default-mold="grid-view">
<cb:mold spring-bean="listViewCollectionBrowserMoldStrategy"/>
<cb:mold spring-bean="gridViewCollectionBrowserMoldStrategy"/>
</cb:available-molds>
</cb:collection-browser>
</context>
Just copy and paste in your new extension in the file <myextension>-backoffice-config.xml and remove the line correspondent to the grid view (for removing the option), eg.:
<context type="Product" component="pcmbackoffice-assortment-collection-browser" >
<cb:collection-browser xmlns:cb="http://www.hybris.com/cockpitng/config/collectionbrowser">
<cb:available-molds default-mold="list-view">
<cb:mold spring-bean="listViewCollectionBrowserMoldStrategy"/>
</cb:available-molds>
</cb:collection-browser>
</context>
Do ant clean all, and after HAC update in your extension.
Ps.: You can apply these changes through orchestrator to see the result in live, before create the new extension.