I cut down our Euonymus japonica microphylla to the ground every few years and it regrows, no problem.

I would cut the long stems back to a few inches into the body of the bush, just above a pair of leaves. Come spring, it should send new shoots out in all directions, and you might want to pinch the tips out of the thickest and most vigorous of those to prevent a repeat next summer.

That doesn't look like Euonymus microphyllus to me, because the leaves look too large - it looks like one of the other E. japonica varieties, and if it is, these get very large indeed, so keeping it small will be a bit of a nightmare. You'd be best off seeking out Euonymus microphyllus instead, which at least only slowly gets a metre high and wide. All respond well to cutting, but best done in spring (well, usually, but this year, we don't seem to be having a winter at all).

It looks like that planting is pretty crowded for the types of plants that you have, and that bamboo in the background is likely to try to eat the the rest of the grouping, even if it is a clumping type. The others will respond to bonsai techniques to keep them small, but that will turn into endless labor over time. You might want to consider redesigning and transplanting, in the appropriate seasons.

Thanks everyone and Thankyou Tugbrethill, it's something I've thought about a lot but I like maintaining everything micro and bonsai like, the garden is only in its second year but we shall see, Thankyou for the answers I have trimmed the main stem back

Bless you, Bealyuk! For some, it's labor...for others it's a lifeline. You might want to consider one of the many kinds of miniature bamboo, possibly confined to a bottomless nursery container sunk in the soil.