Is this maybe the regex that you're looking for? There may be more elegant ways, but this seems alot easier to decipher. 
s/(,42\b|\b42,|^42$)//g;

[download]
Update: Added the case for a single element of just "42" per Not_a_Number's and Perl Mouse's suggestions. Good catch; that's what a quick solution gets me, I didn't test all cases. I should note too, the position of 42 at the end of the list is important, so the regex engine doesn't try and grab that without looking for commas first.
Update: Perl Mouse makes a good point here: 505335 on boundary checking too, so I made another mod to the regex to take that into account as well.
Update: Ok, this is it; no more updates. One catch w/ boundary checking is in the odd case of "41,I like 42 things, 43", which would replace the "42", but shouldn't. I think in the context of this question this is really isn't an issue and \b works fine. To combat that though, I just hit what really was being addressed, "^42$", where its the only element.
Update: lol. I give up; one more. Perl Mouse pointed out partial number matches, so the boundary checking is back in a modified form. He has this below too. I only mod it here so in case someone takes a quick glance, they can get my best version (with the help of mightier monks than I) instead of my regex version from like 4 iterations ago.

-jbWare

In reply to Re: Howto strip 42 from comma separated list using s/// by jbware
in thread Howto strip 42 from comma separated list using s/// by ady

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