Ideally this should not happen. But if you apply too much paint during the spray painting session, you risk runs in the paint. Removing runs differs between bristle and not bristle paint. Waterborne paint I treat here as non bristle.
To remove runs in spray paint, cut them away from the rest of the surface. The hard thing here is to make it a flat cut following the rest of the surface. If your surface is even, then the easiest way I know of is to use a Veritas flush plane. (I have bought a lot of powertools, sanders, routers, ... I use them a lot, but more and more, to get the assembled material to really go together, I use hand tools. Especially hand planes and scrapers are fantastic to make things flat and even).
Anything that allows you to do the cut along the surface works. A razor blade for example is also good, but harder to grip. A chisel works if you can get it in place, but it is longer, and the handle might prevent you from being horisontal. The advantage with the Veritas flush plane is that it is quite easy to get it aligned with the surface, and then keep it aligned with the surface while cutting. It will detect some uneven parts you might not notice yourself. And it is not likely to deviate downwards into the paint due to the forces from the paint while trying to cut it off.
It is theoretically possible to use sandpaper. But in the very same way as when sanding wood flat, sandpaper is good at making things smooth, not at making things flat. If there is a hill or a dent, it will follow it. If you sand a little too much, you are likely to cause a small dent. Sanding only the protruding part of a paint drop is not easy to do.
The target when removing a run is to cleanly cut off the run from the rest of the surface. If it succeeds, you are left with a flat surface, where the run is replaced with a non gloss paint. If you have enough paint beneath to buff it, you could buff it and have saved the piece. If there is not enough paint left to buff it, you now have an even surface. Sand it, and then apply a new coat of paint on the piece.