Do they know that they are in danger of being fired? The prospect of unemployment is definitely something that can focus the mind.
If you haven't given them a clear indication that their performance is unacceptable and that a continuation of said unacceptable performance will get them canned, do so. Hopefully in some of the cases, you'll see an improvement. Others you'll probably have to fire.
Some of them know, couple of them quite new hires who started with a very bad performance (I've no clue how the hell this is possible! - maybe my fault)
Also I openly discussed all these issues and invite them to find a solution all together. So far not going well.
In some cases after being informed of possible serious consequences their performance got back to normal for a month or two then cycle repeated itself. So maybe every 2 months I should remind them that they very close to getting sacked.
If you haven't given them a clear indication that their performance is unacceptable and that a continuation of said unacceptable performance will get them canned, do so. Hopefully in some of the cases, you'll see an improvement. Others you'll probably have to fire.