Hi Pete, glad to see you're still around!
I didn't see that article when poking around their site, but was able to get to it via Google search. I have added the lines to robots.txt, which they claim they obey, so let's see what happens (they say it may take up to two weeks). Thanks for the pointer!
According to my cPanel Error Log, no one seems to be blocked on the one IP address I added. However, tons of accesses from 46.229.168.* (a Dutch site belonging to SemRush) are being blocked, although my Visitor Log still shows me being pounded on every second or two by SemRush. In addition to the check for User Agent, I added a check for Referer "semrush.com", which seems to have triggered the Dutch site denials. Also, (fingers crossed) I am not getting clusters of PHP errors since the Dutch site was denied.
Once the robots.txt entry shows its effectiveness, I'll try removing the other .htaccess bans one at a time.
I wouldn't mind SemRush visiting me once in a while to collect stats for whatever they do, but hitting me every second or two is excessive. To rub salt into the wound, it's the same forum/stats page over and over, and it doesn't change all that much! If they had any brains, they'd notice the lack of change, and cut back on the visit rate.