At work, I have had 5 requests in the last 2 months by the same person asking if we are interested in selling one of our domains. The first time, I asked management and was told no interest as this domain is central to a new marketing/branding strategy. The first 3 times, I tried to explain this. The last 2 times I have responded with: "There is no interest in selling this domain. Please stop contacting me about this." The first request was to the current whois contact address. This last one was to the contact that originally setup the domain (I don't know where they found this address).

I am getting frustrated with this person, but wanted opinions on the next reasonable step. Should I report this person to their ISP? I don't feel it right to report them as spam because the address is there to discuss the domain.

...Seems you've done (almost) all that could be expected of you. I'd suggest one last attempt to stop this person by explaining why you believe her/his behavior has crossed the line to harassment and that her/his next contact of you on this subject will be followed promptly by your filing a complaint with her/his e-mail provider. If you can find them, refer her/him to the TOS of her/his e-mail provider and/ or ISP to support your position.
...First contact was probably not spam but seems like it certainly has crossed over into the category of "abuse."

I think I would try ignoring them for a little while.

It started as a legitimate business/commercial contact. You've given the parties the courtesy of a flat but polite refusal, repeatedly and severally. You don't need to do any more.

If they persist, repeatedly, with essentially the same request, no new or specific matters to put which they might reasonably imagine could warrant or tempt consideration (they, presumably, don't know your company's plans for the domain), then I think it could be classed as abuse.

It could be considered that all of us 'here' have something of a 'hair trigger' when it comes to abuse. Probably best, when it comes to commercial matters and our employers' interests, to err a little on the conservative side just in case we might stray outside the norm and maybe create some sort of liability.

Of course it could be they *do* know (somehow) of the plans for the domain and are trying for some sort of leverage. Can't imagine what that might be, offhand, but in any event badgering you wouldn't help progress it. At the very least it would ensure nobody your end forgets to renew registration.

Does the domain have much SEO value? http://estibot.com/ etc.

[edit - heh, just checked the value of an abandoned domain from my previous work. When it was active it had a higher $ value (= more recognition) than the company's 'main' domain. I pointed that out to them but they just abandoned it anyway. Looks like some squatter/speculator has grabbed it (but no web page) and its value has dropped to base = $20 in estibot terms. Oh well, what's 'web presence' really worth anyway? More than the cost of renewal and hosting a single web page, I would have thought.]

Can't rely 100% on a "website appraisal" like what the above website offers, value often depends on the target audience as well which such sites don't take into consideration.

Completely agree, and it is a good point to make. There are different appraisal sites, mostly using variations of the same/similar algorithm but they tend to give slightly differing results, none of them with any particular validity and mostly being some sort of "come-on" to invite a transaction of some sort, mostly involving dipping into the domain-squatting 'business', it seems.

I don't take any notice of that, I was just using the 'valuation' as a shorthand or comparative ranking that looks at (mostly) hits/visits to the website pages and external links to it. Those are good things to have an impression of without it always being necessary to have the details - having a single figure for each (virtually an index) for comparison of different domains (preferably 'valuations' from the same appraisal site) is helpful. Since it is a notional (though somewhat spurious, as you point out) dollar figure, that makes it easy to talk to management about as they find money strangely exciting and at the same time comforting, even if you qualify it as 'funny money' (as I did to them). But 'money' doesn't always compel them, bless their little white cotton socks. They mostly stick to what they know. And that's fair enough. A (business) domain must be worth more than the money paid to register it, however that value is measured.

I've had the same pursuers for several years.

The REAL remedy is say "YES"

Yes, you ARE interested in selling.

Your price is $1,000,000

Offers will immediately stop.

Of course, you would probably sell for a million,
so if they come back, maybe they're serious.

:-)

(Adjust the amount up or down in relation to how serious you are about selling. 5-million is not too much to ask if you just want to get rid of the sutor.)