The matter is that my computer has 3Gb of memory ram. Therefore i think that i don't need too much swap memory size, and i really need free space in the hard disk. sorry by my English
Yes. You can change the size of the swap partition, by using GParted. You can install it from the Ubuntu Software Center.
Remember that gparted won't work on a mounted partition. When you are working on the swap partition, you have to unmount it first with swapoff. When you are done, to active the new swap space you do it with swapon.
NOTE: You might want to consider doing this from a live/bootable CD or USB, to be safe. Also keep in mind that anytime you deal with partitions, there is risk involved in doing so.
NOTE: When dealing with partitions, always make sure that a backup is available in case something goes wrong. You may consider Clonezilla
I think that your best option is to lower the "swappiness" value, dealing with the partition table of your HDD can be tricky and dangerous.
run this command
add this line
restart the computer, the kernel will not use the swap, the swap will be mounted but unused.
you can set this between 0 and 100, the meaning of this option is about the percentage of use of the swap space, 0 = never use the swap.