I was praying it would happen, and my prayers were answered? 
Seriously, it seems that a selected few HTML tags are allowed by the forum software, and one of them is the strikethrough element. What you do is surround the text you want to strikethrough with <s> and </s>, like this: “I’ll <s>pray for</s>think about you”, which gives you “I’ll pray forthink about you”.
Edit: on further experimentation, it seems that the forum software supports it natively, just use square brackets ([ and ]) instead of < and >. Like this: [ s]strikethrough[ /s] => strikethrough (had to add a space after the opening square brackets to prevent the forum software to interpret the tags).