Give them access to a static, read-only dump of Wikipedia hosted on an internal prison server. Update said dump once every month.
There's also nothing urgent about the access they get. Articles could be restricted to 1 or 2 years old to further prevent the transfer of timely information.
Its possible that lower security prisoners can be treated differently.
edit: typo
I'm only half-joking, I'm sure the prison authorities would be very concerned with this stuff.
... though using torrents is generally recommended to reduce server load and bandwidth.