SELECT *  FROM dba_tab_privs WHERE grantee = 'PUBLIC' AND owner = 'XXXXXXXXXX'

GRANTEE    OWNER      TABLE_NAME                     GRANTOR    PRIVILEGE                                GRA HIE COM TYPE                     INH
---------- ---------- ------------------------------ ---------- ---------------------------------------- --- --- --- ------------------------ ---
PUBLIC     XXXXXXXXXX ST00001VQR0xGSIqbgUwYKF6yqCA=  XXXXXXXXXX EXECUTE                                  NO  NO  NO  TYPE                     NO 

SQL> create or replace trigger trg_grant
  2  before grant on database
  3  disable
  4  declare
  5    name_list ora_name_list_t;
  6    n         binary_integer;
  7  begin
  8    n := ora_grantee (name_list);
  9    for i in 1..n loop
 10      if name_list(i) = 'PUBLIC' then
 11        raise_application_error(-20000, 'GRANT to PUBLIC is not allowed');
 12      end if;
 13    end loop;
 14  end;
 15  /

Trigger created.

SQL> alter trigger trg_grant enable;

Trigger altered.

SQL> grant select on t to scott;

Grant succeeded.

SQL>  grant select on t to public;
 grant select on t to public
                 *
ERROR at line 1:
ORA-00604: error occurred at recursive SQL level 1
ORA-20000: GRANT to PUBLIC is not allowed
ORA-06512: at line 8