I have found a few posts about this, but I haven’t seen an answer to how to permanently disable this function. One comment is that the changes are harmless. They are not - as soon as you export the file (to .xhtml, for example), the changes all show up.
The only way I have found to get around this is to open the file, show the changes, toggle the option to turn off tracking and then manually go through the file and delete all the items that are struck-through. I have no use/no desire to keep track of the changes and just want to permanently turn off the function (including the option to disable/remove changes in a file that I receive from someone else). Or, if there is some option to completely wipe the document clean before exporting it, that would be OK, too.