How to prohibit forcing outputs in PLC

Best way is to apply "Password" protection for the plc program to avoid any interference from any unauthorised person.

in the logic you can not prohibit....but in configuration you can deactivate it.

or better put password.......best option.

I would tell you very clearly if i would know which brand you are using.

I have seen in S7300 PLC, the inputs can be forced, but outputs (M coils, Q coils)can not be forced. So it would be possible in S7400 also. But i don't know how to do it.

Safety-critical systems are best hard-wired through keyswitches or lockable isolators and their use controlled under a Permit-to-Work system that encompasses positive isolation.

In case of any force of output coil in the logic, it will result in accident.

If any output fails "on" will it also result in accident?

If this is the case, all the outputs (or power to the outputs) must be run thru a safety circuit to prevent "on state failures" from accidents.