If you use one of these templates, and there are objects you cannot click on and delete, right click on the slide and choose Format Background > Hide Background Graphics to try to remove these objects.
Good Example
Below is an example of a poster demonstrating best practices in formatting and design. If you click on the poster below you can see a larger version.
(used with permission from Colin Purrington's website)
Bad Example
Below is an example of what not to do when designing your poster. If you click on the poster below you can see a larger version. (used with permission from Colin Purrington's website)