Some details would help.

What sequence of commands are you executing? Which window type are you using?

In general, interactive clients like SQL*Plus and PL/SQL Developer tend to run various steps automatically after returning from a database call. In this case it appears PL/SQL Developer is calling dbms_transaction.local_transaction_id to determine whether to light up the Commit/Rollback toolbar buttons. I'm guessing what you'd like to see is a setting to disable this at window level (perhaps a SET command for Command windows), but I am pretty sure there is no such option currently.

What I generally do is put both the query and the call to dbms_xplan inside a single PL/SQL block.

Code
begin
    execute immediate 'alter session set statistics_level = ALL';

    for test in (
        -- Your test query here:
        select * from some_table t
        where  t.some_col = 123
    )
    loop
        null;
    end loop;

    for x in (
        select p.plan_table_output
        from   table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST +OUTLINE +NOTE')) p
    )
    loop
        dbms_output.put_line(x.plan_table_output);
    end loop;

    rollback;
end;

One advantage of this is that you can use native PL/SQL variables such as dates, which aren't supported by SQL*Plus.