Currently it is not possible to remove uploaded GPX files yourself. It is probably best to write an email to the talk mailing list and provide the necessary info (filename, tags, username) and hope that someone with access to the server is willing to remove it.
While adding timestamps to your GPX files by hand is a noble craft :), there are tools that can do that for you. I believe GPS babel can do that for you, but there’s also a Python script specifically for that.