How can i manually disable the k2 plugin from inside the database?

You can locate it under the _extensions table in the database and disable it.
I don't want to pry but why do you need to disable the K2 plugin through the database?

I don't want to ... i have to...I have a site...that we did tranfere it from one server to another.... The database is extremely large(full of spam comments)... the k2 is the problem...maybe some security thing was disable...So after the transfer the site ain't opening...i contact the web hosting company(who did the tranfer) and they said to me that it was a k2 problem.. I want not to loose 10,000 article...! If i disable it i'll loose it?... Btw....are you Greek?

If the issue are the comments you can disable them and clean up the spams from the database. The table is _k2_comments.

I;ve already disable the comments but the forntpage still doesn't open...so they told me that i must disable the k2 plugin. What do you think?..

Is the com_K2 correct? And witch exactly value i change under the params?

I think that you should cleanup the comments table from the database. That is probably your issue. Do that, clean up both site and browser cache and check again.

There is no need to completely disable the whole K2 component.

ok ...i ve already clear the comments.... i'll send a pic to see for your self.. the size of my db....all the finder tables are enormous

I believe you are referring to this table _finder_links right?

I think all of the finder_links_terms tables are too big...!