Did you try really hard? I have succeeded on the first try; just didn't like the result.

I didn't really learn it, I just had a few reasons to block some thought direction (impression rehashing gone harmful), and so learned to block. The basic idea is like that: get any idea you can tolerate - imagining literal word "CENSORED" cast in concrete is a fine starting point. When you notice the "blocked" thing occurs, think the "showstopper" thought. If you have multiple background attention threads, fill them with independent copies of the showstopper. Once this becomes nearly a conditional reflex, the blocked thought will wear away because it is never reinforced in actively operating memory. It will stay in passive long-term memory, of course.

A close technique - replacing any attention thread with something easy to shut down and shutting down - allowed me to shut down inner monologue. I got some performance-boosting consequences out of that. While it is hard to say how it felt in real time (I did this precisely to have some excessively repetitive things done while minimising the experience of doing them), retrospect memories of this are quite uncomfortable.