Your database did already have one or more devices with
mixed data and log. Were they separate, ASE would
have added the new device as data only.
Now, simply remove the logsegment for those devices
you want for data only but are now mixed:
sp_dropsegment "logsegment", ssbb, mydevice1
sp_dropsegment "logsegment", ssbb, mydevice2
This doesn't remove log pages from those devices, but
normal database activity will eventually clear old log pages,
thus removing them.