Disabling OOXML Export
This is a fairly easy process, made difficult by the lack of line-breaking in the configuration XML. In order to accelerate parsing, the entire configuration is a single line.
Once you've mentally parsed that, then find your configuration; it should be in the directory
<install path>/basis3.3/share/registry/ (Linux) or <install path>\basis3.3\share\registry\ (Windows)
Now load the files - writer.xcd, calc.xcd, impress.xcd and search for:
<value>IMPORT EXPORT ALIEN 3RDPARTYFILTER PREFERRED</value>
If this refers to a filter mentioning 2007 - eg. oor:name="Impress MS PowerPoint 2007 XML" or oor:name="MS Word 2007 XML" - simply remove 'EXPORT' from the string, and repeat this in each file for each instance, also search for "Open XML" and repeat the process - there are quite a number of hits, reflecting a number of different variants.
Save, re-start LibreOffice (note you may need to exit the system-tray quick-starter), and re-try saving in OOXML. Notice, you cannot.