NOT RELEVANT AFTER THE MAY 2022 UPDATE
A [Non]Silent Way To Block Users That Have Sent You An FR
THE ISSUE
You can't block a user via MDL Web who has sent you a friend request before and you have rejected that friend request.

The only option valuable on profile is to accept the request, but you can't accept and unfriend the person as you have already rejected the friend request.

Technically it should have returned to normal MDL options to before the person sending that request, so it's a MDL bug ;)
THE HACK
Let's see how we can block the user using this trick, I'm assuming you are unable to find that user through app so adding this extra step of sending a DM!
Step 0 - Send a DM to the user.

Step 1 - Open Inbox and find the user in the SENT/INBOX tab in MDL App and open user's profile.
Step 2- Open the action menu using the three dots.
Step 3- Once the action menu loads, click on the block option. This will block the user.

Step 4 - Open the user's profile on the website to verify.

NOTE - You don't need Step 0 if you can find the user through the app!
Special thanks to @Ackery for letting me user her profile and asking this question on feeds.