It is a difficult question, as I know the answer ... but not the results !
Answer is yes, you can.
It is just a matter of selecting/deselecting which add-on you want. Like above, you need to go to the properties:
From there, go to the DLC tab:
You have all DLCs, the cosmetics DLCs, the tools and gameplay add-ons, and at the bottom of the list (if, like me, you have many/all DLCs, scroll down the list), the 4 expansions in the order of appearance in the game (Mercenaries, Crusade, then Intrigue, then Retribution)
The mercenaries expansion (for me) is not at the bottom but higher in the list.
Just unselect the expansion you do NOT want to play, and Steam will uninstall it immediately (no warnings!). When deselecting, it is better to do it in the reverse order of installation. 1st Retribution, then Intrigue, then Crusade, then Mercenaries. Wait between each expansion that Steam has finished uninstalling it and then proceed in reverse order to the next one. At the end of this process (it is fast), you are with the original game, almost ...
I just tried deselecting the last 3, and 3.1gb was instantly wiped out (and it is reloading now).
In theory, the game should now launch as it is supposed to be played before you install an expansion.
What I do NOT know: Will it work? Do you need to unselect also some DLCs not designed for the expansions you just uninstalled. Or the game will automatically select the DLCs playable with your customized configuration ?
I do not know, and don't want to go too far, as I am in the middle of a tough game, and I would not like to destabilise something ... I am not a "pilote d'essai" ...
Don't forget that the game will not re-install automatically a DLC if you have deselected it manually here in Steam. Keep that in mind if you want to retrieve a full version and you forgot that you de-activated things, and feel that something in your DLCs is always missing : go back to this DLC tab, and check carefully that you have selected all boxes. Re-select everything unticked before. Steam will start re downloading again.