typeset -r     Make names readonly.  These names cannot then be  assigned values by subsequent assignment statements or unset.

       unset [-fv] [name ...]
              For each name, remove the corresponding variable or function.  If
              no  options  are  supplied,  or the -v option is given, each name
              refers to a shell  variable.   Read-only  variables  may  not  be
              unset.

[/tmp]$ {(b=8; readonly b; echo "-$b-"); b=90; echo "[$b]"; unset b; echo "[[$b]]";}
-8-
[90]
[[]]