I don't have that option at all.
The thing is, if you want to delete a form, you simply go to the page where it's located and delete it.

Same problem here!

I have deleted the form on the front-end too and it still there. When I click the Delete Unused form it does nothing and sometimes it delete the form.

It would be nice to been able to force delete a specific form.

Anyone else having that problem ?

Anyone? This is actually happening to me right now. I can't delete any unused forms. Neither Simple forms or the Extended Form addon

5.4.1 seems to have fixed this.

Problem still happening for me in 5.4.1.

Yeah, you're right. I just forgot to chime in with this. One unorthodox workaround is to attempt deleting it with a different browser: if deleting the unused form via Opera doesn't work, try IE or Firefox. Or vice versa. Not sure why it works, but in most cases it did the trick for me.

You're right, I was using Chrome which wasn't working, tried Firefox and it worked.

Wow - what a crazy fix! I tried in Firefox, Chrome and IE, but it wouldn't delete. Then I tried it in Safari (on Windows) and it did!

Different browsers didn't work for me (I'm on a mac and tried chrome, FF & safari). However I copied the db to my local environment (I have scripts to do so) and was able to delete it locally and push it back up. Crazy bug.

Switching from Firefox to Chrome (even though I had cleared my cache) worked for me in 5.4.2

If you have old versions of a page that still contain reference to a form, you can't delete it from the Reports page.

Go to the page, and click edit, Versions, and delete the old versions.

Took me a while to figure this out.

I have deleted all previous version of the page (and therefore every reference on the site to the old form. I have also cleared the cache. Any suggestions on another way of getting rid of these old forms from the Forms Results page?

Thanks to a few other similar posts I worked it out.

Removing old versions of the page that contain the unwanted forms seems to work, however my specific problem was a little trickier...

After I had removed all old versions I still had the problem. It turns out that the unwanted form wasn't on a live page. To find it, I clicked the "Open Page" button next to the form name and it took me there. Deleting that page wouldn't have (I'm sure) solved my problem, however in my case I didn't want to completely delete it because I wanted to keep it for potential future use. Instead, I went into Edit on that form and changed the name of the form to be "NOT USED" (plus a short description).

I hope this helps others who stumble on this.