eshell-kill-process-signals

eshell-kill-process-signals is a variable defined in `esh-proc.el'.
Its value is

(SIGINT SIGQUIT SIGKILL)

Documentation:
Signals used to kill processes when an Eshell buffer exits.
Eshell calls each of these signals in order when an Eshell buffer is
killed; if the process is still alive afterwards, Eshell waits a
number of seconds defined by `eshell-kill-process-wait-time', and
tries the next signal in the list.

You can customize this variable.