Had quite a battle with the fidopoll.bsy flag from 00:40am this morning. Finally got it solved after a reboot and forcing it off. It was still there after a reboot, so stopped everything and killed it, then restarted:
drwxrwxrwx 2 pi pi 4096 Sep 27 11:12 .
drwxr-xr-x 36 pi pi 4096 Sep 27 02:07 ..
-rw-r--r-- 1 pi pi 0 Sep 27 00:40 fidopoll.bsy
-rw-r--r-- 1 pi pi 4 Sep 27 11:12 mis2.bsy
-rw-r--r-- 1 pi pi 4 Sep 27 11:12 mis.bsy
-rw-r--r-- 1 pi pi 0 Sep 27 10:59 mutil.bsy
-rwxrwxrwx 1 pi pi 0 Sep 3 10:48 nodeinfo.now
-rw-r--r-- 1 pi pi 0 Sep 27 02:07 qwkmail.out
pi@npie:/mystic $ ./fidopoll killbusy
FIDOPOLL is already running.
pi@npie:/mystic $ sudo killall mis mis2
pi@npie:/mystic $ ps aux | grep fido
pi 4524 0.0 0.2 4272 1856 pts/0 S+ 11:30 0:00 grep --color=auto fido
pi@npie:/mystic $ s
pi 4638 4636 0 11:31 pts/0 00:00:00 grep mis
total 8
drwxrwxrwx 2 pi pi 4096 Sep 27 11:29 .
drwxr-xr-x 36 pi pi 4096 Sep 27 02:07 ..
-rw-r--r-- 1 pi pi 0 Sep 27 00:40 fidopoll.bsy
-rw-r--r-- 1 pi pi 0 Sep 27 10:59 mutil.bsy
-rwxrwxrwx 1 pi pi 0 Sep 3 10:48 nodeinfo.now
-rw-r--r-- 1 pi pi 0 Sep 27 02:07 qwkmail.out
pi@npie:/mystic $ sudo rm -rf sem*/fidopoll.bsy
pi@npie:/mystic $ sudo rm -rf sem*/mutil.bsy
pi@npie:/mystic $ s
pi 4737 4735 0 11:32 pts/0 00:00:00 grep mis
total 8
drwxrwxrwx 2 pi pi 4096 Sep 27 11:32 .
drwxr-xr-x 36 pi pi 4096 Sep 27 02:07 ..
-rwxrwxrwx 1 pi pi 0 Sep 3 10:48 nodeinfo.now
-rw-r--r-- 1 pi pi 0 Sep 27 02:07 qwkmail.out

OK, that's done it. Proof:

pi@npie:/mystic $ ./fidopoll killbusy

FIDOPOLL Version 1.12 A35

Purging .BSY files for all echomail nodes...

Time to restart and see what happens.
Just restarted mis & mis2 and everything was fine except the attackers tying
up more and more nodes. So created a script for /root:
#!/bin/bash
# block
#cd /mystic
#sudo rm $2.zone #kill old list
#sudo -i #become root
ipset -N $1 hash:net
wget -P . http://www.ipdeny.com/ipblocks/data/countries/$2.zone
for i in $(cat $2.zone ); do ipset -A $1 $i; done
iptables -A INPUT -p tcp -m set --match-set $1 src -j DROP
iptables -L #only for feedback to you

Found I had to do the first three commands manually, then:
block china cn
block russia ru
block india in

All blocked, only one node being attacked for now ;)

--- Mystic BBS v1.12 A35 (Raspberry Pi/32)
* Origin: Mystic Pi BBS bcw142.zapto.org (21:1/145)