There isn't a built-in solution for the forums. However, you can do it with certain browser adblockers, if you're willing to do a little bit of legwork.
Here's a quick rundown of the process, based on the Google Chrome browser and uBlock Origins adblocker (not a recommendation, it's just what I'm using right now).
1. Go to this URL (copy and paste it into your URL bar):
chrome-extension://cjpalhdlnbpafiamejdnhcphjbkeiagm/dashboard.html#1p-filters.html
You should see a window with a text box (this is the uBlock dashboard), and a row of buttons along the top that say "Settings", "Filter lists", etc. with "My filters" being the active tab (click on it if it isn't).
2. In a separate tab, navigate to the subforum that contains the thread you want to hide.
(e.g.
https://www.autos.ca/forum/index.php?board=19570.0 , which is Off-Topic Discussions)
3. Right-click on the title of the thread you want to hide and in the menu that appears, go to Inspect.
4. In the window that appears, you should see a highlighted line that says
<a href...>TITLE OF THE THREAD</a>5. Look at the line above it, it should say something like
<span id="msg_988827">Make a note of the number after "msg_".
6. Back in the tab with the uBlock dashboard, copy and paste the following into the textbox, replacing the number after "msg_" with the number from the thread you want to hide, making sure to not replace any other character.
autos.ca###msg_988827:nth-ancestor(3)7. Click "Apply changes".
8. Refresh the page with the forum topics. The thread you want to hide should now be gone.
Note: this just prevents the thread from appearing the topic list, it won't stop you from going to the URL directly.
If this process didn't scare you off and you'd like instructions for other browsers/adblockers, let me know.