You can press Space to tag a process. The kill command applies to all tagged processes.
There's no easy way to tag a process and its children, but the tree view (t) should list them contiguously.
Depending on how recoll is run, the processes may be in their own process group. If they are, then you can use
kill -STOP -1234 to suspend them all, where 1234 is the process group ID (usually but not necessarily the process ID of the initial process in the group). You can check with
ps -o pid,ppid,pgid,comm -C recoll, then
ps -o pid,ppid,pgid,comm ww | grep -v recoll to see if there are any other processes in the process group. Htop doesn't have an interface to process groups.
If all the processes are called
recoll, then an easier method is to filter the processes by name. In htop, use the filter command, then you can easily tag the processes you want to kill. On the command line, run
ps $(pgrep recoll) to list the matching processes. If you're happy with the list, run
pkill -STOP recoll to suspend those processes.