The editor used in the forum software does not include the strikethrough feature. The functionality can be added through the use of custom bbcode and the placement of an appropriate icon within the editor's toolbar.
Unfortunately, due to the quirks of our hosting provider, I am not able to make those changes myself as they require that custom files be uploaded to the server, which requires that they do it for us and we pay for their efforts (currently approximately $90 per hour with a 2 hour minimum charge).
I have considered installing the latest version of the software on a personal server, making all the tweaks we might enjoy and then zipping the entire thing up and submitting it to our hosts as an upgrade, which they will do for free. Perhaps when the next major upgrade is available.