I remove excess metal. Sometimes metal is soft and you can scrape it of with a utility knife. Sometimes its steel and then you need to use sand paper, or in extreme cases dremel like power tool. Be careful not to take of wood. I've has this problem with quite a few sets.
Treatment of jiggling chess pieces due to uneven base?

Perhaps you could visit your local home improvement center and purchase something like this to use for sanding down the unevenness of the bottom of your pieces that aren't level. I have used this when I weighted a 3d printed set and it worked like a charm. The felts went onto the surface completely flat when adhered with glue with no divots in any of the 40 pieces I felted. I used a two-element epoxy clear gloss resin and some of the pieces developed a slightly uneven surface when hardened so this little sanding disc did the trick rather quickly and nicely to remove the uneven surface quite well. All I had to do was attach it to my power drill and off I went.
https://www.homedepot.com/p/DIABLO-5-in-StickFast-Sanding-Disc-Kit-DCD050KITD01G/202830915
Hello,
it happens again and again that a single figure swings an uneven base and the figure swings about 1 second when touched or placed on a new field. A friend solved this by sticking the whole set of figures with new, very thick felt in such cases. I want to know what other options are available to easily fix these problems. I thought of removing the felt and sanding the base down a bit and then sticking on a new felt. But with what grind? With a file or emery paper. Report, how you solve the problem, I have not found anything here in the forum.