A member has recently deleted her account on my forum; she would like all her posts deleted. Is this possible and if so, how do I go about deleting 5000+ posts from this deleted user? 
Many thanks.

Hello @Geocachingmama,

You can try this : https://help.forumotion.com/t134526-remove-messages-of-a-member

However, I am not sure if it works for members who are now guests.

No i have just ran a test and that system don't work for Guest

Okay thanks @Ape,

If that is the case, the only thing you can do is remove the messages by hand. 5000 is a lot, so just do what you can manage.