I would second the community garden idea. They are all the rage. Anywhere there is a community garden there is a waiting list to get in. It would be a win-win compromise for all. The principle that this is common property would be upheld. Unilateral annexing of common property would not be rewarded by giving it away but nor would there be the unedifying sight of ripping out a productive garden and reinstating a decorative garden that perhaps no-one really benefits from or uses.
I think the next thing to do is to draft a bylaw for use of the community garden. I would suggest the EC allocates plots on a year by year basis to any interested resident. You have to give up your plot if it is not being used or not kept tidy. If demand is high the plots will be subdivided so all units have an opportunity to use part of the garden. Note that some might want only a little corner for a few tomatoes while others might want to grow a great variety and quantity. Have some process for fair and flexible allocation.
You might want to use some OC funds to develop it further with certain one-off costs, say paving between rows, fencing around the whole area, or some screening planting from more formal garden areas. That would be justified as improving a community facility. On the other hand manure/compost/mulch for one’s assigned plot would be at your own cost. Depending on your situation you might prefer a shared compost area that could take kitchen scraps or you might require people to prepare compost within their own unit areas.
I suggest you use this as an opportunity! Tell the person who did this that it was not really done the right way, the OC really cannot tolerate a land grab, but, now that it is done, this person and the OC could make something of benefit to all out of it.
Back in 1976 our OC adopted the following at its first general meeting. The language is a bit dated now but I think it is essentially sound. Community gardens went out of fashion and our early one was disbanded. Recently we tried to reinstate it and it met with significant opposition and did not get up.
“7. The Corporation may set aside a portion or portions of the common land for gardening purposes and permit in writing members to use that land or parts thereof to grow plants or keep animals or birds thereon SUBJECT TO any conditions which the Corporation may impose.
8. (1) Every member who obtains the use of any garden area under the preceding Article:
(a) shall not use the same EXCEPT as a garden and for the purpose of growing plants or keeping animals or birds and shall not grow or keep animals or birds or permit to remain on the garden allotment, EXCEPT with the written permission of the Corporation, any plant or animal or bird EXCEPT a plant or animal or bird grown for food or other domestic purposes PROVIDED THAT if the Corporation considers that a plant or animal or bird whether grown or kept for food or other purpose is noxious or causing a nuisance may require the member to remove the same and the member shall forthwith comply; and
(b) shall at all times keep such garden allotment well cultivated and tended.
(2) If the member shall be guilty of a breach of any conditions subject to which a member has been granted a garden allotment or of any of the conditions in the preceding subsection the Corporation shall without prejudice to my other rights or remedies of any kind, be it liberty by its servants, agents or workmen to enter into the garden allotment and to remove and/or destroy the animal or bird or plant planted or remaining in the garden allotment in contravention of such condition and to remove any fertilizer, herbicide or insecticide or other chemical prohibited in such conditions and/or to apply any substance to the allotment on any part thereof to any plant growing thereon to neutralize or counteract the effect of my such fertilizer, insecticide, herbicide or chemical.
(3) Without limiting the generality of this article, the Corporation may grant garden allotments subject to conditions relating to access by other members, use of water and drainage and fees chargeable for use of garden allotments.”