• Update existing records in a table - trim leading and trailing spaces:

  update your_table set
    lan_id = trim(lan_id);
  

• Use database trigger which will make sure that only trimmed values are stored into a table:

  SQL> create table test
    2    (lan_id varchar2(8));
  
  Table created.
  
  SQL> create or replace trigger trg_biu_lan
    2    before insert or update
    3    on test
    4    for each row
    5  begin
    6    :new.lan_id := trim(:new.lan_id);
    7  end;
    8  /
  
  Trigger created.
  
  SQL> insert all
    2    into test values ('123 ')
    3    into test values (' 456')
    4    into test values (' 678 ')
    5  select * from dual;
  
  3 rows created.
  
  SQL> select lan_id, length(lan_id) len from test;
  
  LAN_ID          LEN
  -------- ----------
  123               3
  456               3
  678               3
  
  SQL>

Update the table to remove all spaces from the existing values in the lan id column.

UPDATE your_table
SET lan_id = regexp_replace(lan_id, '[[:space:]]*','');

to not let any blanks or space to be entered in the field