Is it possible to send senders of leaflets an invoice and claim the money via the small claims court, in a similar way to how Ikea sends a parking charge if you have been on their property (car park) too long? Junk mail senders are trespassing on my land and ignoring a polite notice not to post anything that is not personally addressed. Would it not be right that we as individuals can submit an invoice to the sender and then take them to court for non-payment after a suitable period of time?
You can certainly send them an invoice but it's unlikely the sender would pay up; and if the sender would challenge your (legal) claim you'd almost certainly loose the case. Ignoring an anti-junk mail sign is rude, and you'd hope that most senders respect such signs, but there's nothing in current law that enables you to enforce anti-junk mail signs.
A better option is to send individual senders a trespass notice. The main difference with your suggested approach is that you'd enter a legal agreement with the sender – when the agreement is breached (i.e. when the sender enters your property to deliver junk mail) you can then send them an invoice. This approach has been tried with some success (follow the above link for a detailed explanation and example notice letter) but legally it seems questionable. I can't really say much more on the legality – I'm not a solicitor.
You could perhaps stick a huge notice on your door, with a text explaining that depositing unsolicited mail on your property is considered an offence. I'm not sure if such a notice would constitute a legal agreement – if you're seriously considering going down this route you should ask for proper legal advice. I suspect they'd agree that a formal trespass notice would have more chance of success, but then again I'm not a legal expert.
Finally, there are other (easier!) things you can do. For instance, why not send any unaddressed junk mail you get back to the sender in an unstamped envelope, with a note asking the sender to respect 'No Junk Mail' signs in future? It will annoy the sender, cost them a little bit of money, and it will make them aware of the issue. If enough people take such action even the most dodgy junk mail senders would very quickly start respecting anti-junk mail signs.