Negative weights are not available for the beginner interface.
Terms can also have negative weights.
A negative weight for a term penalizes any documents that contain that term. If you want to discourage documents containing a term, set that term's weight to a negative number. The negative weight will detract from the document score for any documents containing that term. However, documents containing the term still could appear in the results, because other terms may contribute to the document score. For example, after looking at your search results, you decide that you want to discourage documents that contain ther term power plant .