The EC could write (or just talk) to the real estate agent whose vehicles are blocking access. Most bylaws/rules/articles will have a catch-all rule that says a unit/lot owner must not interfere unreasonably with the reasonable use and enjoyment of the common property by other residents. Blocking access to common property facilities would count as unreasonable interference. In the ACT (perhaps elsewhere is similar) there is a specific provision in the Act for a resident to ask the EC to issue a ‘rules infringement notice’. I would start with asking the EC to ask the estate agent to please observe the sign. Move on to heavier action if that does not work. Ultimately the state tribunal could give orders that the common property facilities not be obstructed. Failure to comply would be a legal offence.