Since this thread was started eBay still have not implemented a means for buyers to set their preferences that would allow them to opt out of receiving offers from sellers - and I doubt if eBay ever will make such an option available, as from eBay's perspective it probably seems like a good idea in that it may convince people to buy something they had initially decided not to bother with, even though they do not appear to have considered the fact that many people will have deliberately clicked off of the listing as a result of reading or seeing something in the ad that put them off of the item - but as with many other aspects of this site, I've just had to adapt to these ever frustrating changes in order to continue using the site.
Although there is currently no way to block unwanted offers from sellers, what I've now started to do is to try to use the system in such a way that it actually works against itself. What I've noticed since this thread was started is that the vast majority of offers I receive from sellers are for items I've only looked at, but have decided not to add to my Watch List. Therefore, if there's an item that I might be interested in buying if the asking price was a bit cheaper, nowadays I usually just view the item but deliberately hold off of placing the item on my Watch List. If the item subsequently shows up on the My eBay page, showing me that I've received an offer from the seller, then so long as the drop in price is acceptable, as opposed to marginal, I sometimes end up buying the item.
If you use this method properly you can, in some cases, get even more money off of the item that might otherwise have been the case. I recently saw an item that I was interested in but it looked to be slightly too expensive for my liking, so I clicked off of the listing without adding it to my Watch List. Sure enough, less than a week later I received an offer from the seller, the price having been reduced to an acceptable amount. In that instance eBay were also running a discount offer on items from certain sellers, so I decided to review the offer and see whether the option came up to apply the discount code. It did, so I applied the discount code before checking out, and ended up getting a further discount on the seller's discount offer!
At the end of the day I can understand the frustration that people may feel with receiving unwanted offers, but rather than allowing it to stress you out, try to do as I've explained above and find ways to use the system against itself. The only other alternatives at present are to: A) decline the unwanted offers in the manner that I described when I previously posted on this thread, B) ignore the unwanted offers until such time as they expire and then delete them, or C) stop using eBay and buy items from other sources, such as other online selling platforms or from High Street retailers.
