How to protect from competitors?

Hello,
I am hitting a market with some tough players. I would like to secure myself once I start getting the top positions. So far I've been ranking by using simple PBNs, profiles, blog comments and forum entries. I am trying to block robots.txt of my PBNs via htaccess in order to make them at some point not visible for my competitors.

Anyhow what steps can I take to secure myself from the competitors trying to destroy me?

If you block robots.txt via .htaccess, it's like not having any robots.txt at all. In general, you should block backlink crawles of tools like MOZ/OSE, ahrefs and so on via .htaccess only. Two advantages: robots.txt is just a recommendation to crawlers, no real block. In addition, everybody can see what you try to block. .htaccess is a real serverside block, no third party can see what exactly you are blocking.

In general, you can't protect yourself 100% from evil competitors. Take negative SEO as an example. Short term attackes will be recognized by Google and won't work, but if someone keeps attacking at a low level over a long time, it will work at some point. That's something you have to deal with in some niches, just keep building new sites, so the ones going down are constantly replaced....or don't enter this niche.