Delete the win partion in the partition table. Depends on your partion layout, if the old win partion is conjunctive to another linux partition, for example / or /home, you could use gparted to resize the linux partition to use the new free space. Otherwise, you could create a new linux partition and linux file system, modify your fstab to mount it, for example as /data, /backup, etc. Finally, modify your grub conf to remove the windows entry and update grub.