ImageMagick should use the disk when memory cannot be allocated but there might be a bug in the library. Can you create a small program that I can use to recreate your issue?
When an exception is raised the memory will be cleaned when the MagickImage is disposed. Are you sure the MemoryAllocationException is raised by ImageMagick because it is out of memory?
When an exception is raised the memory will be cleaned when the MagickImage is disposed. Are you sure the MemoryAllocationException is raised by ImageMagick because it is out of memory?