Other than that, you should never set the volume size & the LUN size to the same value - any volume should always have some free space in it.
One of the ways you can get out of the trouble, is to increase the size of the volume & also you may consider thin provisioning, i.e. setting volume space guarantee to none (this plus some other settings, like snap autodelete are explained in aforementioned blog post).
Also bear in mind ONTAP is (under normal circumstances) not aware of data being deleted within a LUN.
If your LUN is the same size as your volume even with no space reservations and no snapshots the ONTAP FlexVol will be full because the LUN is using up the space in the volume. This is ok as long as you don't want to take a snapshot of that volume. The Windows host should see the free space inside the LUN so the host should be able to write inside that LUN even though the FlexVol says it has no space.
Now, starting with SnapDrive 6.1 for WIndows (only for Windows), if you thin provision your volume, SnapDrive reclaim blocks for the volume so that you are only consuming the used space in the volume that is being used in the LUN.