just try to take it offline.
If there is any active transaction, by default oracle does not allow you to take the concerned RBS offline.
Anyhow, that IS NOT RECOMENDED. try in a test machine.

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this query may help you
SQL> ed
Wrote file afiedt.buf

  1  select  r.name,
  2     l.sid,
  3     p.spid,
  4     nvl(p.username,'no transaction') username,
  5     p.terminal
  6  from    v$lock l, v$process p, v$rollname r
  7  where   l.sid = p.pid(+) and
  8     trunc(l.id1(+)/65536) = r.usn and
  9     l.type(+) = 'TX' and
 10     l.lmode(+) = 6
 11* order by r.name
SQL> /

NAME                                  SID SPID      USERNAME        TERMINAL
------------------------------ ---------- --------- --------------- ----------
PRODR1                                              no transaction
PRODR10                                             no transaction
PRODR11                                             no transaction
PRODR12                                             no transaction
PRODR2                                              no transaction
PRODR3                                              no transaction
PRODR4                                              no transaction
PRODR5                                              no transaction
PRODR6                                              no transaction
PRODR7                                              no transaction
PRODR8                                              no transaction
PRODR9                                              no transaction
SYSTEM                                              no transaction

13 rows selected.