Can't drop SCN indexes, resource busy. help !

Hi
Re document ID 375401.1 regarding high SMON thrashing, I tried to drop the indexes and rebuild them as advised using the procedure:

connect / as sysdba

shutdown immediate;

startup restrict;

drop index smon_scn_time_scn_idx;

drop index smon_scn_time_tim_idx;

create unique index smon_scn_time_scn_idx on smon_scn_time(scn);

create unique index smon_scn_time_tim_idx on smon_scn_time(time_mp);

analyze table smon_scn_time validate structure cascade; 

I got this for the "drop index" statements:

ORA-00054: resource busy and acquire with NOWAIT specified

Understandable if smon is thrashing I suppose. Tried waiting a while, I know smon wakes on schedule and maybe it hasn't finished the previous scan before the next one starts. But no luck. No matter how long I wait, it's continuously hitting that. This is a dev database that has been lurking in a corner and I hadn't noticed this was happening. Appears to have been that way for a while now.