You can decide whether to allow a lock or unlock with a hook. A hook is more or less a callback which allows to react on repository events and sometimes in order to prevent them.
Hooks are stored in your $(svnrepos)/hooks. there should already be templates you can use as cue for how to implement it. best you read the comments in pre-lock.tmpl and pre-unlock.tmpl. They explain how it works and what to do. This system is a bit complex but powerful.