Disable copy constructor

I have a class:

class SymbolIndexer {
protected:
  SymbolIndexer ( ) { }

public:
  static inline SymbolIndexer & GetUniqueInstance ( ) 
  { 
    static SymbolIndexer uniqueinstance_ ;
    return uniqueinstance_ ; 
  }
};

How should I modify it to disable code like:

SymbolIndexer symbol_indexer_ = SymbolIndexer::GetUniqueInstance ( );

and only allow code like:

SymbolIndexer & ref_symbol_indexer_ = SymbolIndexer::GetUniqueInstance ( );