Disabling Activity Status values

Summary

I just want to disable values in Activity status but I cannot untick it.

Content

We have our values for the Activity Status field and we're not going to use the existing values so we are disabling the old values. But we cannot disable nor delete the old values on application composer (disabling via lookup) or via manage activity standard lookup task. Is there any way we do to disabled it? Hope you help me. Thank you in advance.