[Baertram]

Just an update:

I got my wanted behaviour now by removing the item's context menu entries.
If you remove the context menu entry the ralting keybind will be automatically removed too.

I did this by pre-hooking the contexnt menu actions ZO_InventorySlotActions, function "AddSlotAction":

--======== RIGHT CLICK / CONTEXT MENU ==========================================
    --Pre Hook the right click/context menu addition of items
    ZO_PreHook(ZO_InventorySlotActions, "AddSlotAction", preAddSlotAction)

The function called:

--Pre Hook function for adding right click/context menu entries.
--Attention: Will be executed on mouse enter on a slot in inventories
--AND if you open the context menu by a mouse righ-click
--To distinguish these two "events" you can use: if self.m_contextMenuMode == true then
--true: Context-menu is open, false: Only mouse hoevered over the item
local function preAddSlotAction(self, actionStringId, ...)
 
--------------------------------------------------------------------------------
--Get the parent control and bag + slotIndex
    local parentControl
    --Chracter equipment, or normal slot?
    if (not ZO_Character:IsHidden()) then
        parentControl = self.m_inventorySlot
    else
        parentControl = self.m_inventorySlot:GetParent()
    end
    --No parent found? Abort
    if parentControl == nil then return false end
 
    local bag
    local slotIndex
    --gotta do this in case deconstruction, or player equipment
    local dataEntry = parentControl.dataEntry
    --case to handle equiped items
    if(not dataEntry) then
        bagId = parentControl.bagId
        slotIndex = parentControl.slotIndex
    else
        bagId = dataEntry.data.bagId
        slotIndex = dataEntry.data.slotIndex
    end
    --case to handle list dialog, list dialog uses index instead of slotIndex and bag instead of bagId...?
    if(dataEntry and not bagId and not slotIndex) then
        bagId = parentControl.dataEntry.data.bag
        slotIndex = parentControl.dataEntry.data.index
    end
 
--------------------------------------------------------------------------------
--CHECKING THE CONTEXT MENU ENTRIES HERE:
--If you want to hide this context-menu entry you need to return TRUE, otherwise FALSE
 
    --Destroy item?
    if    actionStringId == SI_ITEM_ACTION_DESTROY then
 
        --Only execute if context menu is visible?
        if self.m_contextMenuMode == false then
            return
        end
 
 
 
        --This here will hide the "Destroy" contetx-menu entry and the keybinding in the keybind stip as well!
        return true
 
    --Add item to crafting station, improvement, enchanting table
    elseif actionStringId == SI_ITEM_ACTION_ADD_TO_CRAFT or actionStringId == SI_ITEM_ACTION_RESEARCH then
 
        --This here will hide the "Add to craft" and "Research" context-menu entries and the keybindings as well!
        return true
 
    --Trade an item, mark item as junk, attach item to mail, sell item
    elseif actionStringId == SI_ITEM_ACTION_TRADE_ADD or actionStringId == SI_ITEM_ACTION_MARK_AS_JUNK
       or actionStringId == SI_ITEM_ACTION_MAIL_ATTACH or actionStringId == SI_ITEM_ACTION_SELL
       --sell in guild shop
       or actionStringId == SI_TRADING_HOUSE_ADD_ITEM_TO_LISTING then
        --Is item marked with any of the FCOItemSaver icons? Then don't show the actionStringId in the contextmenu
 
        --This here will hide the "Add to trade", "Add to mail", "Mark as Junk", "Sell" and "Add to sell list" context-menu entries and the keybindings as well!
        return true
 
    --Equip an item
    elseif actionStringId == SI_ITEM_ACTION_EQUIP then
 
        --This here will hide the "Equip" context-menu entries and the keybindings as well!
        return true
 
    --Unequip an item
    elseif actionStringId == SI_ITEM_ACTION_UNEQUIP then
        --Only execute if context menu is visible?
        if self.m_contextMenuMode == false then
            return
        end
 
        --Only for character items
        if( bag ~= nil and slotIndex ~= nil and bag == BAG_WORN) then
            --Update the character window after item was unequipped
            zo_callLater(function() checkMyCharacterWindowNowFunction(bag, slotIndex) end, 200)
        end
        return
 
    --Debugging:   Gives you the action string Id if you hover an item
    else
        d("actionStringId: " .. actionStringId .. " = " .. GetString(actionStringId))
    end
end

If you return true the context-menu entry and the relating keybinding will be hidden.
If you return false the context-menu entry and the relating keybinding will be shown/kept.
If you just return (nil) nothing will be changed. Maybe the same as returning false.