If you find that a negative comment might still be relevant to other people, then we suggest keeping them and responding in a helpful manner. In most cases, this can help build trust with leads and allow for a level of transparency in your community. People love to read these threads, and occasionally seeing a real person responding ads brings a humanistic quality to the advertisement.
Generally people on Facebook can tell the difference between negative commenters and genuine comments, and it will be up to you to judge if the comment should be left or deleted.
If you delete negative comments, we also recommend blocking the person after deleting their comment, as this will keep them from repeating the same behaviours. Taking these kinds of actions will not affect your ad in any way, as blocking and deleting comments or account are not directly linked to your ad.