I don't seem to find way to block competitor sites in the Google Custom Search product though I'm able to in normal AdSense ad units.I've tried to search in Google here and there but could not find answer to this.Is there any alternative to Google Custom Search?
Blocking specific ads in the Google Custom Search
google adsense;google custom search
If you're referring to blocking competitor ads, Google Site Search does provide this feature:  Google Site Search vs Google Custom Search  Here's how: Disable AdsIf you mean blocking competitor sites entirely from appearing in search results, you could try using the Google Custom Search API:The Google Custom Search API lets you develop websites and programs to retrieve and display search results from Google Custom Search programmatically. With this API, you can use RESTful requests to get either web search or image search results in JSON or Atom format.