not recommended as there is no hook location for this, so you would need to hack the core files (which means you need to do this after every upgrade)
but if its important to you 
 
source/rankings.php
this is previous <:
	
	
	
		Code:
	
	
		$TMPL['multiple_pages_p'] .= $TMPL['url_tail'].'" rel="nofollow"><</a></li> ';
	 
 and this is next > :
	
	
	
		Code:
	
	
		$TMPL['multiple_pages_n'] .= $TMPL['url_tail'].'" rel="nofollow">></a></li> ';