Negative Points come from shops that you accepted and then canceled, had unassigned because you did not do it, or had the shops declined. If you have questions about the specific circumstances surrounding your drop or decline you should contact the MSP that posted the shop in question.
iSecretShop does not issue or take away shop specific points.
You can also answer Opinion Polls and take courses as they are available on iSS Prep. These will earn you points.