بسم الله الرحمن الرحيم
It is not clear on what ground the management has banned any other person to lead janazah prayer other than the imam. However, the ruling is that if the wali of the deceased person is better than the imam of the locality then he has more right to lead the janazah prayer otherwise the imam of the locality is entitled for it. However, the wali should give permission to the imam and let him lead the janazah prayer.
وتقديم إمام الحي مندوب . بشرط أن يكون أفضل من الولي وإلا فالولي أولى الخ . (در مختار)
Allah knows Best!
Darul Uloom Deoband