The Board of Aldermen and the Planning Board on Monday will hold a joint public hearing on a proposal to enact a temporary moratorium on medical marijuana dispensaries through June 2014. The aldermen approved a ban on medical marijuana dispensaries locating in Melrose in January, just two months after Massachusetts voters approved a referendum legalizing marijuana for medical purposes. In March, the state attorney general ruled that a Wakefield zoning bylaw banning dispensaries in that town was illegal. Since then, several cities and towns have turned to temporary moratoriums to allow them more time to craft regulations about where to locate a dispensary. The hearing will be held at 8 p.m. in the aldermen’s chamber at City Hall.