I don't use Core, but somewhere in your <Game>/VGUIAssets/Shells/<Addon>/Windows/ directory there should be a file that represents the reactions window.
In that file there should be a section which defines the reaction entry; there may be multiple definitions, for Rescues, Sympathetic, etc. For each entry definition there will be a bit of code which determines mouse behavior for the element.
Code:
<Action event="_begin_drag" senderName="#SetAsSender">
<Notifications>
<Message>ABILITYSLOT_DRAG</Message>
</Notifications>
</Action>
You'll want to comment out that bit, so that it looks like this:
Code:
<!--<Action event="_begin_drag" senderName="#SetAsSender">
<Notifications>
<Message>ABILITYSLOT_DRAG</Message>
</Notifications>
</Action>-->
The
begins commenting out, and the
ends it. If there are any other comments
within that bit, you'll need to delete them; you can't nest comments.
You'll do the same for any other elements (weapons, bags, etc.) which you don't want to be able to drag. Just find their respective .xml files and search for any definition which has ABILITYSLOT_DRAG in it.
Hope that helps.