which type of japonica? Laurel, anemone etc. japonica is the second part of the name so we need the first name to know which plant it is.

Sellio, Could it be Kerria japonica with pretty yellow flowers in spring and early summer? Sounds like the big one I dug out two year s ago. If so, it wasn't a mammoth job but I do still find the odd runner which I chop off. They grow too big for my garden and shaded a whole bed.

I suspect the plant you mean is Chaenomeles japonica - my mother in law always called in japonica. If it is that, unfortunately this plant suckers prolifically, and is encouraged to do so by hard pruning. Your only recourse is to cut down all stems to about an inch from the bottom, then cut or drill into each one and apply SBK brushwood killer, neat, without spilling on the ground, to the cut areas, and cover with something (upturned pots, stones, whatever). If some of the stems are thin and weak, try to expose some soil to reveal a woodier, thicker part and treat that instead. This treatment is actually more effective if carried out on central, woody roots, but I realise that digging too much isn't an option, so you can try treating the stems in this way.