Actually the problem is not stuck screen pixels, but dead screen matrix drivers (since it's complete vertical columns of emitters that are missing, not individual emitters). A pixel massager wouldn't fix this in a month of Sundays.
Unfortunately the matrix driver electronics is almost certainly an integral part of the display module, and can't be replaced separately. In the UK, prices for 20inch screen modules are around £600, not including the fitting (not easy in an iMac).

Bad news (costly repair) or good news (excuse to buy a shiny new Mac)? Your choice! :O)

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)