The bot group in phpBB helps you identify known bots based on their user agent string, you can give them different permissions and phpBB also makes minor changes to the pages. For example usernames are no longer linked to the profile. Removing them doesn't accomplish anything because then they are anonymous users. This would be counter productive because pages are going to have links to resources you do not want any bot accessing. For example if bot/guest tries to access the profile page they will get a "You do not have permission..." error page, it's just a waste of server resources and removing those links prevents bots from trying to follow them in the first place.
If you want better control over legitimate bots look into using robots.txt, typically if they are identifying themselves with user agent string they will adhere to robots.txt directives. They can be blocked entirely with robots.txt... keep in mind what I said was "legitimate bots". The ahrefs and semrush bots are marketing bots and can be denied access through robots.txt, for most sites they serve no useful purpose. Not sure about Amazon but DuckDuckGo is legitimate search bot, their search focuses on user privacy. You can of course research each bot and see what it's for.
Last but not least be aware the user agent can be spoofed. For example there is browser extensions to switch to user agent to Google bot and others. You can then brows your forum to see what the pages are like as Google sees them.
“Results! Why, man, I have gotten a lot of results! I have found several thousand things that won’t work.”