Unfortunately, there's currently no sure way to block/prevent a Job Scheduler from being overriden. Two "soft" ways to "enforce" this come to my mind:
1. "Enforce" these as a good development practices/rules within your organization.
2. Use the System Runtime Context for that purpose - make sure that only a handful of apps/FWs are included in the S-R-C and remove others. You can switch the "Disable automatic updates" option in S-R-C and that will prevent any new apps from being added automatically to it. This unfortunately puts the burden of managing the runtime context on your System Administrators (similar to ASYNCPROCESSOR Requestor Type in 8.1 and 8.2 versions). Please note that Job Scheduler are still going to override the OOTB platform ones if they are defined in any of the "whitelisted" apps/FWs that are included in S-R-C but you should have the situation a bit more under control.
@Avinash.Haridasu Can we try to Mark the Job Schedulers in FW layer as FINAL as after a rule is marked as final, no one can create a second rule with the same visible key in any RuleSet other than the RuleSet that the first rule belongs to. If the RuleSet version of a final rule is locked, you cannot create a second rule with the same visible key in any version of any RuleSet, except a higher version of the RuleSet containing the final rule.