People are free to try offer better deals and services though often times they'll likely be much worse.
Offering better deals in this area almost always means offering horrible services.. I don't think it is fine to do this, especially not when the user was already hired. If you allow this, then you essentially allow users to do this out of spite; slippery slope here and there, you reject someone's entry in a bounty because of them being a farmed account: They attempt to get you ("legitimately; by offering "better" services") fired.
Even if you lose business temporary if you provide a good enough service the word will get out and you'll continue getting business.
No. In traditional industries, sure. For bitcointalk ICOs, no. A lot of the ICOs don't really give a damn about the negative effect (spam, farming and whatnot) and would go for the cheapest option that just marks which addresses need to be paid.