Jack, Thanks. Your suggestion following on from George's suggestion makes sense. However, bearing in mind that when installing apps on an iMac we are supposed to do it from the Apple App Store on the iMac, rather than from the third party website, I tried to do that. I can't find any Malwarebytes app in the App Store. I can find it on the malwarebytes.com website. But I'm not sure if it's a good idea to download it from the third party website and fear I may cause myself another problem.
Maybe it's not available in the App Store because the MacOs version I'm using is so old (MacOs v 10.13.6) and is no longer supported by Apple. The reason I haven't updated that is due to another problem. (Adobe Reader won't work on later versions of MacOs and I need Adobe Reader. Perhaps I should start a new question on that issue as it's a separate problem).
If you have any further suggestions about deleting the Malwarebytes icon, other than updating the operating system, I'd be grateful to see them here. I'm surprised there's no way to delete an icon off the toolbar by dragging it to the trash bin. That was how I deleted the Malwarebytes app, I think.