I always notice that Ubuntu start using swap, as soon as the RAM usage is around 60-70%. How to configure it so it should wait so RAM usage became 80-90%. And is there a way to empty the swap, as soon as the there is space available on the RAM, other than swapoff manually?
The answer to your question how, is "no". Linux is extremely good at memory management. You could never hope to compete with it. And you wouldn't want to unswap just because you had free RAM. Because Linux knows what parts of the memory is most useful. In other words; you'd reduce performance drastically by manually moving unused memory from the harddisk and into RAM.
See the section titled "What is swappiness and how do I change it?" on this page:
But, I also agree with Jo-Erlend Schinstad. If there is no urgent need to change the default behavior, it is best to leave it alone. However, I see that the article itself recommends a value of 10, while the default is 60. So, changing to the recommended value of 10 should take you the way you want to go (less swapping).
Also, part of your original question is, how to empty swap. swap is emptied every time you reboot.