Remove any NAT or firewall device between you and the internet. Such systems trend to keep sessions open, even if they are disconnected at the client.
Here our rules:
REJECT tcp – * * 0.0.0.0/0 0.0.0.0/0 tcp dpt:873 flags:0x17/0x02 #conn src/32 > 1 reject-with tcp-reset
ACCEPT tcp – * * 0.0.0.0/0 0.0.0.0/0 tcp dpt:873 flags:0x17/0x02
That is all, we don´t have any timer or other limitations.