Create a custom search provider. Search URL should be like:

https://www.google.com/search?q=%25s+-ai+-reddit

You can just keep stringing the additional terms that you want excluded. I'm sure there's some limit but you probably won't hit it

Edit: apparently either lemmy or boost doesn't like percent signs. Or URL encoding for that matter. The first part should be (percent character)s, and the plus signs should be (percent character)20. Though the + will probably work as-is depending on how your browser does things

Better use the actual -site:domain function to filter out by domain directly. You can in example search everything about Reddit, without results from Reddit itself:

reddit -site:reddit.com -site:redditinc.com -site:reddithelp.com

I don't know of a way with Google, but with Kagi, I can block, penalize, or promote domains from my search results.