When more than one user (legitimate or otherwise) connects to the wifi network, they share the available bandwidth. This causes the throughput to a specific computer to be reduced if another PC is using the available bandwidth at the same time.
What they should NOT do is cause the SPEED of the connection to drop. If you have an 802.11b network at 11Mbps, then EVERYONE should "see" the network speed reported as 11Mbps. Similarly for 54, 108 etc.
If you see a connection speed reducing, then this is a result of interference. One or more network cards is swamping the signal for others. Other wifi networks and other radio devices (e.g. mobile or wireless phones, bluetooth devices - even microwaves and door bells) can also affect the signal strength.
If you are troubled by illegitimate use of your wifi network by others, then you should apply WPA security and MAC address filtering as others have mentioned.
You can also use fixed IP addresses rather than DHCP, and configure the wifi router (if it supports it) to only allow the IP addresses you have assigned to access resources (mainly the internet).
SD