I assume you mean the swirly patterned, ridged stuff (like plaster) - if so, I'm not sure that you can remove it. Most people I know that had this finish on their ceiling, have had it reskimmed with a smooth finish. If it's textured paint, you can get a remover from DIY stores - not cheap, but then what is these days?
I am sure ipek is correct. I have seen it sanded down using a flat bed orbital type sander but this is a seriously difficult task, and maybe not worth the effort. If it is very rough artex you could remove the worst this way and then "re-skim" what is left with new plaster.
I one of the DIY programmes on TV they said that a wallpaper steamer would do the trick. I've never had the need to try it [no artex] but if you do I suggest that somewhere inconspicuous is tested first.
We had our ceilings re artexed a while ago. The bloke who did it had a metal scraper about 6" wide attached to, effectively, a broom handle. This meant he could stand on the floor and scrape the length of the ceiling. This then leaves a lot of small ridges, which you can then artex over again, or if you want it smooth it has to be skimmed with plaster or artex to a flat finish. Not a job for the faint hearted!!
I have used X-Tex Textured Coatings Remover on my whole house to remove artex from walls and ceilings.It is brushed on and left for a couple of hours. In some cases you can leave it on overnight if the artex is painted. The just scrape it off. It also works on pre 1985 artex containing asbestos, as it keeps the artex wet and does not let any fibres into the air. There are no fumes and it is totally safe so it wont burn your skin like other removers or steam method. I bought it from decorating direct.