Excess charges in a metallic conductor always try to go as far as possible from each other or in other words, they try to lower their potential energy. The charges try to do this by moving to a region of low potential energy (electrically speaking).
For example, leaking in a capacitor means that the negative charges move from the negative to the positive plate. This way the charges reduce their potential energy and finally there is no net charge present in the cap plates. This would give rise to a current for a short time. And sparks are nothing but ionization of the surrounding material (air in this case). This happens when the electric field is strong enough to overcome the dielectric barrier. Sparks can be seen when you short a cap.
Now back to your question,
As per the details provided in your question, the metallic charged sphere will want to leak charges to a region of low potential energy
(say a negatively charged entity or a neutral entity or the ground).
But the charges have no way to do so if the material surrounding the sphere can ideally provide sufficient insulation and also you have to make sure the supporting entity on which the sphere sits can ideally isolate it from the ground.