Yes, thay can, they will, they probably are interfering.
Ad-hoc wireless network uses the same power level as client-to-AP (at least for now, there's another 802.11 spec coming to allow power adjustment at the client .... like cellular phhones).
If you have a WLSE, you can identify the ad-hocs and (I believe) kill their sessions automatically. The WLSE (or other wireless security device) send each client a disconnect that drops their session every time it comes up.
You still have some interference, becasue the clients are out there searching for connections, but once it becomes understood that ad-hoc won't work, hopefully the users will either associate to your APs or shut down their wireless connection.
This is where a good usage policy come into play. It should be understood that if employees bring in their (wireless) systems, that they aren't allowed to ad-hoc (or only ad-hoc on specific channels, using specific (permitted) ad-hoc SSIDs).
You may also want to add that rogue APs are not permitted (at least without permission).
The (wireless) network administration needs to have some constraints for security: To be able to tell which wireless activity is "friendy" (but, maybe ignorant), and which activity constitutes illegitemate interference or an attack, so it can be dealt with.
Without a policy, you have no legal basis for taking action against offending clients / users.
To get back to the original question: yes, any other activity on the wireless channels you are using will likely cause interference and negatively impact your throughput.
Good Luck
Scott
