Thanks for your enquiry and am sorry to hear of your plight.
As regards their right to put up such signs and whether you can prevent this from happening, you have to look at the agreement between you and the Housing Association. This is of course set out in your Tenancy Agreement.
I am afraid there is not normally anything in a typical Agreement preventing them from putting up any such signs- the only covenant given by a Landlord which governs what they can and can not do as a Landlord is the covenant for quiet enjoyment. This covenant will state that the Landlord "covenants to give you, the Tenant, quiet enjoyment of the property" (ie if you have noisy neighbours/ experience harassment from the Landlord constantly gaining access to your property, then you have the right to take action against the Landlord).
The erection of the signs would not amount to a Landlord's breach of this covenant. Indeed, most Agreements will include a clause giving the Landlord a right "to deal with the building as they deem necessary" and therefore legally, I am afraid there is nothing you can do.
All you can do to appeal to their better nature and hope that they agree to remove them.
I am sorry this is not the answer you were looking for, but it sets out the legal position.