How can I secure these inadequate/leaning deck supports?
There are lots of different types of post to beam connectors.

Like this:


However, the real weird thing is that you don't really have beams. They are just supporting the joists with a bunch of random posts on concrete deck blocks...

If it were me I'd lose all of those and put in a couple (or more) beams under those joists and put some posts under those.

Not in that order, though. Leave the posts in, put in some proper beams and posts, and then lose the crappy ones you have there.

Below is an example of a similar height deck I did. There are two beams under the joists of this deck, and four piers (two for each beam). The beams are the ones with the white flashing tape on top. You can clearly see one in this image, and how I attached it to the joists.