Dullcote and Testors Lusterless Clear are two spray on sealers I've used for over 20 years and for the most part they work well.
The one major drawback of spraying sealer on your figures is that it reduces all the paint surfaces to an even uniformed level of "dullness". This can be helpful when you're trying to reduce or eliminate shine caused by paint, however the better option, as Einion suggested, is to use paint that naturally dries to a flat finish.
Sometimes you want your figure to have some shine on it (ie. exposed skin, polished leather, metallic surfaces) so the spray on sealer is not always the best option. One way you can avoid killing the shine is to mask the shiny part of the figure before you spray.
I often finish my figures with a light dusting of pastel chalk to simulate dirt and dust, and the chalk needs to a fixed to the surface with a sealer to prevent it from blowing or rubbing off. A quick spray of Dullcote usually does the job.