ibdata growing too fast (no replies)

I have inherited a running MySQL production server (5.5.19) which seems to be running out of diskspace rapidly (5GB free).

MySQL datadir = /data/mysql5519_prd/

/data/mysql5519_prd/ibdata is now 42GB, 2 days ago 40GB so growing fast.

All database tables already use separate .ibd files, those are present next to the .frm files.

my.conf contains these relevant innodb properties
innodb_data_home_dir = /data/mysql5519_prd
innodb_data_file_path = ibdata:512M:autoextend
innodb_log_group_home_dir = /data/mysql5519_prd
innodb_buffer_pool_size = 512M
innodb_additional_mem_pool_size = 20M
innodb_log_file_size = 100M
innodb_log_buffer_size = 8M
innodb_flush_log_at_trx_commit = 1
innodb_lock_wait_timeout = 50
innodb_file_per_table

So, both innodb_file_per_table is present AND a tablespace file.

This seems superfluous, and I want to get rid of the large ibdata file.

I know that it's normally only possible to get rid of this ibdata file by dumping and reimporting in a clean mysql instance.

However, since innodb_file_per_table is already present, is there a simpler way?