false advertising and consumer rights in Belgium

I'm looking for a little advice on what constitutes false advertising in Belgium and what my rights are as a consumer. Groupon.be seem especially bad. For example, I bought a voucher about 2 weeks for a set of 16 Mach 3 Razor blades for 11 euros (relative to the normal price of 26 on the shavesavings website.) My credit card was charged and I received the voucher, only to get a mail from Groupon 3 days later saying they were retro-actively cancelling the offer due to a "publication error". Are they allowed to do this? Can I demand that they actually offer what they advertised?

Secondly, Groupon also seem to have a consistent policy of advertising discounts larger than they actually are. For example, today they are advertising a 50% discount on a lesson to fly ultralight aircraft (99 Euros instead of 199), but if you go to the Aérocerfontaine website the normal price for what they are offering is actually only 105. Isn't this kind of practice illegal? Can I demand that I get the discount they are offering?

The same applies to an offer they had for a professional UV (teeth) bleaching last week, being advertised for 199 instead of 490, but on the website of the European Estetics Center the price was only 390, or 290 if the appointment was booked for January. They have now, however, increased the "normal price" to 490, but wouldn't they have had to sell a certain amount at 490 in order to call it a discount in the first place?

Thanks in advance for your answers. I've e-mailed Groupon twice, but have not had a reply in over 2 weeks (beyond the standard template that they have received my mails.)