As they're conifers, they're not likely to regrow from the stumps, depending on the variety (Yew might) but the only way to get rid of the stumps is either to dig them out, or use a stump grinder on them - or pay a man with a stump grinder to come and do it for you. Otherwise, they'll rot down over time, will take a few years.

You could try painting the stumps with neat Glyphosate or SBK stump killer , this will speed things up but it will still take a long tome to degrade and breakdown the wood .

urgh conifers!!! I (and 3 friends) attempted to dig out a leylandii conifer stump 3 years ago with zero success!!!

maybe cut it short and let it rot if you can?? iv planted a small herb garden around mine and made it into a mini feature! it does take time though.....mines been cut down 5 ish years now and its still a sturdy peice of wood..... although i had some smaller conifers felled around 8 years ago and it only took 4 years or so before there was zero trace of those!!