The best way I think is to go to 'Advanced Search' (i.e. not just the normal drop-down default box, but the link at the bottom of it) and then you have the option to search by poster name. Put the user you want in there, leave the search words themselves blank (unless you know something specific you're trying to find) and select threads started by the user (rather than all posts by user). That's it (sounds more cumbersome written out than it is, trust me!) - you will then get a list of reports started by the user name you were looking for. Moomba also has some very nice reports for example from memory.