Cheeky blighters. Obviously not a bona fide double glazing contractor, just cowboys.
Have you been able to check the details of sale from an estate agency or look up details of their lease on the Land Registry? If you have recently bought your flat then you will be pretty familiar with any requirements of the freeholder, if it is not jointly owned by the leaseholders, and of maintenance agreements.
What I would do, if you have one that is, would be to put the parasol base, garden umbrella and chairs outside the doors so it prevents them from being opened easily to access the garden, while not totally preventing access In case of emergency. I liked the bike chain idea but could be an access issue for emergencies.