Thanks for using
Notezilla .
I assume you are using the Windows app. If you are using the Android/iPhone app, then the below markdown suggestion will work.
If you are using the standard editor inside a sticky note then the right click menu will provide you with strike through formatting option. The option is available under one of the sub-menus.
If you are using the markdown editor then you can surround the text using ~~.
Example: ~~Sample text~~
You can also select a text and press Ctrl+K to strike through in Windows app.