The way this is working is that rather than sending askubuntu.com to opendns to convert the name to an IP address, you are sending simply askubuntu. instead. Of course there is no askubuntu., and if their DNS server was doing what it should, it should send an error code back to you. Instead, whenever there is no domain name matching what your computers send, it is returning the IP address of its own search service. (184.108.40.206) is what I just received. Ads there are probably how they pay for the service they offer.
There are ways to block this address, but there's no reason to believe they won't just change it. And, if you did block it, you would have your browser just spin and spin waiting for the blocked site to get back to you.
Your browser won't and can't know the difference.
I'd suggest instead that you use the DNS server provided by your ISP. You are paying for it so that's probably a reasonable thing to do.
Another approach is to use google's public DNS servers 220.127.116.11 and 18.104.22.168. I've heard they consider DNS server problems to be a threat to their business so they decided to provide their own DNS service as a public service. I suppose the question is whether you trust them. Their privacy web page indicates they are sensitive to privacy issues, but you should decide whether their policies are adequate for you, and whether you believe that they will adhere to them. Here's the page.
Here's an article with a list of public DNS services. I personally use my ISP's with one of google's as a backup. I'd research level 3 if I didn't want to use my own ISP's, opendns', or google's since their business is bandwidth rather than ads or content.
If you are OK with your ISP's service but it isn't completely reliable, maybe you could list it first and opendns or another open service listed second. That way the public service only get's your lookups (after a couple second delay) when your ISP's server is down.