On the forum?
How about inserting an image of strikethrough text from a Word document?
Create document with strikethrough text in MS-WORd, and leave it open on your monitor.
CTL-printscreen to save your screenview to clipboard.
Open MS-Paint, and CTL-V to paste the screen view.
Crop and size the image and upload it to an image hosting service.
Insert the link into your forum message using the little mountain icon.