click on the conversation . slide up to the top or first one .
in the top right it will say leave conversation .
click that and you have a choice of not receiving any more from that user or accepting more from that user .
whichever choice click leave conversation that's it.
I believe you can do multiples as well.
To delete entire pages, go into conversations when box appears bottom left hit show all, then hit the little selected box top right, then bar box appears across bottom hit select all, then choose action then hit leave conversations, then choose to accept future messages or Ignore future messages, then hit save. If you have multiple pages make sure your on the page you wish to delete.