Is there a way to block users from installing applications on their Blackberry Device?

Yes, you can block 3rd party applications via an IT Policy.

Thanks...

Do you know how to edit and create IT polices on the BES? If not, and if you are the administrator, please take a brief minute to read the Administrator Guide PDF that comes with every BES download you have (or available on the blackberry.com knowledge base).

If you do know about IT Policy items, then it's under the Security Policy Group, listed as 'Disallow Third Party Application Downloads'. You may also choose to disable the use of Application Loader through the Desktop software; this one can be found under the Non-Grouped Desktop-Only Policy Group.

I cannot seem to find the Non-grouped Desktop-only Policy Group in the IT policy. Where is it? Please help. Thanks...

It's the second section in the IT Policy editor.

Is it possible to block installation of only certain 3rd party applications? or is it just all or nothing thing?

If you want to block specific apps, you'll need to first configure them for wireless pushing from the BES. Then you can create an Application Policy for that application and set its disposition to Disallowed. Apply the Device Software Configuration to the users you want to block and voila.

Or you can white list them this way which I assume is how you want to do it, Then your users have to contact you if they want any software installed and you can do it wirelessly bam 4 hours later it shows up.