>> I want to rebuild it because del_lf_rows/lf_rows ratio exceeds 50% for this index.

What are you going to acheive with this?
Nothing.		
The index will go the same state as it was before easily.
Rebuilding the index is Not going to help with anything at all.

If you want to release the extents, Move the lob table ( along with lob index) to another tablespace and bring it back;
or
just move it to same tablespace ( if you have enough space).

mag@mutation_mutation > create table test (c1 blob);

Table created.

mag@mutation_mutation > select tablespace_name,index_name from user_indexes
  2   where table_name='TEST';

TABLESPACE_NAME                INDEX_NAME
------------------------------ ------------------------------
USERS                          SYS_IL0000013450C00001$$

mag@mutation_mutation > alter table test move tablespace users lob(c1) store as lobsegment (tablespace users);

Table altered.