Starbase blocking is rife in the early game: the minors drop starbases deep within your ZoC and often lock out huge lumps of your assets because you can't build any of your own bases near an existing one; in extreme cases they can block chunks of your home system preventing you from setting up your own economic bases (for example).  The AI also seems to cheat and is aware of the location of resources well beyond its own explored region of space.

I had a star base down taking resources and LOST them after a Minor dropped his near mine and snagged resources. I did not know this was possible.

Indeed.

In fact, it seems strange in general that minor races can send constructors and survey vessels etc. to other star systems but not attempt to colonize or even reach those other star systems themselves...

In my B5.3 game I resorted to buying up a minor's constructors as they were built to prevent the problems outlined above (minor's bases prevent my econ and mining starbases in my ZOC). Usually I could trade a tech or two for the minor's constructor, so this did not cost a lot of gold. Then I used those constructors to build econ and mining starbases for me in positions of my choosing. It may be a long time before those bases get boosted, but at least it kept the minor at bay. 

In another game, the minor still snuck a mining base into my ZOC that prevented my doing an econ SB for Earth. That time I was able to actually buy the minor's SB with techs and gold. Then I destroyed the SB and immediately created the econ SB I wanted for Earth with my constructor. It was after that experience that I started trying to trade for minor's constructor if they were close to my territory.

I like the suggestion above that minor's SBs be limited to some set distance from their homeworld.

That's how it was in GC2--Minor races could only settle or construct bases within the sector their homeworld was in.

   I'd like minors to be unable to place a starbase where it overlaps one of your (any other races) planets or starbases, and to be willing to sell once their starbase does overlap one of your (major's) planets.

  Also, a relations drop if one race's starbase overlaps the planet of another race (more for majors than minors).

If minors respected ZoC from the get-go because, you know, they're minors and don't want to start antagonising folk, then that might be enough.

Something needs to be done if the devs don't want minors to be instant automatic targets every single game.

My point being that minors could have reasons to exist that don't make them automatic invasion prospects.  With the current system of them spamming starbases and sometimes even locking you out of your own systems the game is forcing you to take these little sods out ASAP, and without any mercy whatsoever.