I know who you mean (name ends in 275). I wondered,if, like another poster with a similar record, he/she had a browser which doesn't make it easy to reply to his/her own threads, but in the case you refer to the two answers were comments on his/her own thread rather than answers which shows at some stage he/she has a browser which allows this.
But I think there is no reason to ban such users just for failing to say thank you or answer queries. I suggest are all free to choose whether to help or not help these posters.
However there may be a case for discouraging multiple posting or consistently ignoring requests to provide proper clues, letters etc