You can remove entries from ResearchGate only when you created them yourself. When it was added by the system, you can remove it only from your profile. See also https://explore.researchgate.net/display/support/Reviewing%2C+featuring%2C+and+editing+your+research , e.g. the chapter "Why are there duplicate research items on my profile?"
As outlined in the useful link provided by Wolfgang R. Dick ,
to remove the item from the search, click on the item and claim authorship so that it is re-added to your Researchgate profile. Then go to your research items and click on the title of the entry you would like to remove. You will see a small blue arrow pointing down on the upper right side (next to the add full text option). Click 'remove' on the drop-down list.
After this the research item should not appear even when you search.