This is somewhat similar to what C.Fe. said, but with slightly different approach.

I couldn't see it in the original post, but perhaps there is a way to measure the magic energy levels of wizards. Multiple sensors throughout the prison, including each single wizard cell.

When it is time for meals, exercise, or any other time where a prisoner must be allowed out of their cells the sensors in the individual cell must read under 0.1 kiloWhizatts (or whatever a low unit of magical energy is) or the door simply doesn't open. From their cells, they can then channel their energy through a conduit which allows them to drop their levels and get out of confinement.

This would keep the prisoners that are around other prisoners all around the same low levels of energy, and those who chose to store their power are essentially willingly starving themselves and staying in solitary confinement, preventing them from endangering the prisoners who do comply with the rules.

If those wizards' magic energy levels are detected to rise to dangerous levels, then the wizard guards are sent to painfully extract the magic from them.

The energy can be used to power the facility as well as creating batteries/ammunition for resale to keep the prison self sufficient.

This is somewhat similar to what C.Fe. said, but with slightly different approach.

I couldn't see it in the original post, but perhaps there is a way to measure the magic energy levels of wizards. Multiple sensors throughout the prison, including each single wizard cell.

When it is time for meals, exercise, or any other time where a prisoner must be allowed out of their cells the sensors in the individual cell must read under 0.1 kiloWhizatts (or whatever a low unit of magical energy is) or the door simply doesn't open. From their cells, they can then channel their energy through a conduit which allows them to drop their levels and get out of confinement.

This would keep the prisoners that are around other prisoners all around the same low levels of energy, and those who chose to store their power are essentially willingly starving themselves and staying in solitary confinement, preventing them from endangering the prisoners who do comply with the rules.

If those wizards' magic energy levels are detected to rise to dangerous levels, then the wizard guards are sent to painfully extract the magic from them.

The energy can be used to power the facility as well as creating batteries/ammunition for resale to keep the prison self sufficient.

Edit: Now that I see that magical energy can be measured, but for more accurate reading one needs to have needles in them, I'm envisioning somewhat of a permanent prison bracer containing the prisoner's information, which monitors the wearer's energy levels. If the low enough it will act as a key to give the prisoner access to shared facilities.

In the event that a prisoner gets to a shared space and somehow manages to convince enough prisoners to transfer their power to them and increase their power, it would trigger an alarm on the bracer. If it gets to critical levels before the wizard squad could intervene, it could:

a) Trigger the pain spell failsafe in the device or b) Teleport the wearer to a special detainment area where Wizard guards await. (if that's possible with your magic)