User quotas

Hi,
I'm giving my users the ability to upload photos to their own album and I've implemented a quota system so they can only upload 10Mb of photos (using nodeapi validate for image). Does anyone know how I can make Acidfree mass import take notice of this or how I can implement something similar.

I've noticed that acidfree calls node_invoke_nodeapi and I can hook into this, but I can't seem to make the process stop at this stage - I can recognise the limit is being breached but not stop the upload.

Thanks for any help
Andy