UNDEF[INE] variable ...
where variable represents the name of the substitution variable you want to delete.
Deletes one or more substitution variables that you defined either explicitly (with the DEFINE command) or implicitly (with an argument to the START command).
To undefine a substitution variable named POS, enter
UNDEFINE POS
To undefine two substitution variables named MYVAR1 and MYVAR2, enter
UNDEFINE MYVAR1 MYVAR2