Presumably the sign is advertising the sale of one of the apartments in the building. If so, it has been put up by an estate agent who is acting on behalf of the owner of that apartment, one of your fellow-members of the management company. Your options are:
- take it up with the owner concerned; or
- take it up with one of the directors of the management company, and ask him to take it up with the owner concerned.
In either case, the owner concerned can be asked to arrange to have the sign removed.
The management company may well tell you that there is a practice of allowing owners who wish to sell their apartments to advertise the sale of the apartment in this way. It's obviously good for owners in general to be able to sell their apartments as advantageously as possible, and allowing temporary signs of this kind seems to be common. If that's the view taken by your management company then they won't ask the owner concerned tor remove the sign. If you're not happy about that, your only avenue for seeking change is to work, as a member of the management company, to have the policy changed and the rules on signs strictly enforced. That may involve working with other owners who feel the same way as you; it may involve seeking election to the board of the management company so that you can have a more direct influence on the policy around signs.
If the management company does ask the owner to remove the sign, there's no sanction for having put it up, other than having to remove it. If the owner doesn't remove it, the management company can ask whoever they have retained to do the property management and maintenance to remove it, and the rules of the management company probably allow them to invoice the offending owner for the reasonably-incurred costs of doing this, But the cost is probably negligible.
There's no sanction at all for the estate agent involved. They're not a member of the management company and are not bound by its rules.