You can try asking a process to round up its dead children, but unlike the quit signals, the number varies by platform. For most Linux users it's -17, but using the text version -CHLD is probably a better choice (unless you're on a really old system that absolutely has to have a number, in which case check the local documentation.)

If it's a well-behaved process, that can do away with the need to kill it. In other cases, there might be some kind of restart mechanism built in that can be called instead - assuming sending it a SIGCHLD doesn't trigger that behaviour anyway.

Case in point, the Cinnamon DE has at least a couple of ways to restart it, and at least one of those gets rid of its zombie child processes. It's fairly rare that I need to do that, and I haven't tried sending it a -17. I might do at some point.