Eek, only 3 bits of advice I can give and I'm not sure how helpful any of them are!
1.If you can't trust yourself to spend syns on chocolate without going over then don't buy any. Maybe it would be easier to completely cut it out....rather than just having a little taste and then being like a chocolate monster lol!
2. Only buy in small amounts of chocolate, maybe keep 3 freddo's in the house? Then when they're done, buy 3 more. Don't keep more in than what your syns will allow.
3. Portion the chocolate, at the start of each week divide chocolate treats up in to freezer bags of however many syns you choose to have and use them to ration your chocolate intake for the week, hopefully it should discourage you from eating in to another days syns as it's clearly sectioned and would also leave you short on the next occasion.
Whatever you decide to do...good luck! Hope you can overcome it, I rarely have chocolate now as when I do I become a fiend!