Stack Exchange allows users to both upvote and downvote posts, but how this is implemented right now is a double-edged sword.
If a user received too many downvotes in a period of time they will be blocked from posting.
Some users, aware or unaware of this, will downvote any post they disagree with. This doesn't mean the post was made with poor intent or effort, but simply that they don't share the same view.
The thing is that, for example on the Unix & Linux Stack Exchange, the ratio of downvotes I'm getting myself is extremely high compared with those on Stack Overflow or Ask Ubuntu in the same topics.
On those sites I have quite good reputation, where in Unix recently it got so low I can no longer post questions.
I suspect some users there are purposefully downvoting anyone they disagree with as a means of silencing them. They are taking advantage of the capability of the downvotes to censor.
And there's nothing you can do about it. Since all your replies are legit, there's no way you could improve them and unlock your account. The ban is, in practice, permanent.