Hello Julien,
Unfortunately dissociating records in N:N sub-grids is not supported out of the box. There are a few ways to do this however, but they may require some custom work.
Have you considered using a fetch like this?
<fetch distinct="true" mapping="logical" output-format="xml-platform" version="1.0">
<entity name="gw_contact_gw_industry">
<link-entity name="contact" to="contactid" from="contactid" alias="aa">
<filter type="and">
<condition attribute="contactid" value="@cid@" operator="eq" />
</filter>
</link-entity>
<link-entity name="gw_industry" from="gw_industryid" to="gw_industryid" alias="bb">
<attribute name="gw_name" />
<attribute name="gw_industryid" />
</link-entity>
</entity>
</fetch>
Otherwise you could create a custom grid column that calls a custom web service that will handle the disassociation for you.
Regards,
Clinton