How large is it? If not huge just cut it down leaving about shoulder high trunk to use as a lever, then dig it out

Or just spray it with Glyphosate & wait

welcome to GOY Kayberry.

how big is the tree?
you have several options.
easiest is to cut it off at ground level and then 'paint' the stump with SBK root stump killer. available in most garden centres and hardware stores.

if it is really small then dig it out.

or you could ring it. about 6" above the ground gouge out at least an inch of bark all away round the trunk so that you are hitting the harder wood in the middle of the tree. this is how rabbits/deer cause tree death by eating the food carrying tubes.

As the roots wont get any food they will die. you wont know until next spring when the leaves should open if you have been successful or not.

Agree, the key factor is how big are the 'young' trees? You can snip them off with bypass hand pruners. You can pick up a tree saw at the hardware store, designed for this very purpose. A chainsaw will cut through the tree like butter and is really fun to use. Hire somebody to do it for you.

Welcome to GOY, Kayberry. These seedlings spring up all over my garden, often in places where digging them out is tricky if not impossible. I keep a wary eye out for them and douse them in weedkiller as soon as I spot them, and aim to give them a weekly dose to ensure they succumb. We have dogs so the seedlings usually get a plastic bag to wear as well to deter inquisitive canines ingesting any weedkiller.

I would go with Grandad's solution. If you cut it off at shoulder height, you can dig the soil away from around the trunk and rock it backwards and forwards and it will expose the roots which you can sever. If you leave the stump in, it might interfere with future plantings. If did this with some quite tall firs and it worked well for me.