declare
begin
for c1 in (select count(1) cnt from dba_users where username='RAMAMOHAN') loop
begin
if 	(c1.cnt = 0) then
execute immediate 'create user ramamohan identified by abc123 account unlock password expire';
else
execute immediate 'drop user ramamohan';
execute immediate 'create user ramamohan identified by abc123 account unlock password expire';
end if;
exception 
when others then
dbms_output.put_line(sqlerrm);
end;
end loop;
end;
/

select username, to_char(created,'mm-dd-yyyy hh24:mi:ss')  from dba_users where username='RAMAMOHAN';

USERNAME                       TO_CHAR(CREATED,'MM
------------------------------ -------------------
RAMAMOHAN                      05-22-2009 15:29:58


This script also does the same job as the above script, without cursor loop and if condition.

whenever sqlerror continue
--whenever sqlerror exit
begin
begin
execute immediate 'create user ramamohan identified by abc123 account unlock password expire';
exception
when others then
dbms_output.put_line(sqlerrm);
end;
execute immediate 'drop user ramamohan';
execute immediate 'create user ramamohan identified by abc123 account unlock password expire';
exception
when others then
dbms_output.put_line(sqlerrm);
end;
/

PL/SQL procedure successfully completed.

select username, to_char(created,'mm-dd-yyyy hh24:mi:ss')  from dba_users where username='RAMAMOHAN';

USERNAME                       TO_CHAR(CREATED,'MM
------------------------------ -------------------
RAMAMOHAN                      05-22-2009 15:18:00


Happy scripting

References:
http://sqlanywhere.blogspot.com/2009/03/ddl-meets-if-not-exists.html