Manual:$wgNonincludableNamespaces

Access: $wgNonincludableNamespaces
Use this to deny inclusion of pages from specific namespaces.
Introduced in version:1.10.0 (r19934)
Removed in version:still in use
Allowed values:(array of numeric namespace IDs or namespace constants)
Default value:[]

Details edit

Pages in namespaces in this array cannot be used as templates. Elements must be numeric namespace IDs or namespace constants. Among other things, this may be useful to enforce read-restrictions that may otherwise be bypassed by using the template mechanism.

$wgNonincludableNamespaces[] = 3000;

or

$wgNonincludableNamespaces[] = NS_CUSTOM;