You just keep pulling them. Ideally, if the plant sets seeds, you make sure to pull it out before that. Otherwise, you keep pulling until the roots have exhausted themselves. You need to be persistent.

It would help if you use Preen in the planter. The weeds are constantly spewing seed and it's constantly sprouting new weeds. Try to get ahead of it with pulling and Preen and don't let it become infested again. You'll probably need some tweezers.

You have various different weeds there. Some, larger leaved ones, including that tree seedling, will be easy to pull. But you also have an Oxalis and Euphorbia maculata. Hard to pull and tenacious. Personally I'd not fight that battle but leave them and just remove the larger leaved egregious ones. As long as you don't allow the Oxalis and the Euphorbia to escape the trough they're not really worth the battle imo. Both are prostrate mat formers and will not bother nor overtop the Equisetum

I am truthfully a weed-a-phobe. The danger and why I couldn't stand to leave them is fear of their thousands of seeds "impregnating" any nearby landscape beds. I would rather they just be gone so I could sleep peacefully.