Preferably without involving direct game files modification. I know Borderlands 2 save editor, but a downside to it is that any extra unspent skill points added are removed, presumably down to how many one should have. Optimally, I have exactly enough skill points to max out everything, and no more. I have heard of the character switch glitch which is typically instructed involving split-screen, but I play on PC.
Here's what I did:
1) Use a Quick Change station to respec your character, so you have a large(-ish) number of unspent skill points.
2) Use Cheat Engine to find the location of unspent points in memory.
3) Lock the memory location while I added points to everything I wanted, then unlock it when I'm done.
(In theory, if the issue is "I don't want the game giving me any more points once I'm maxed out", you could keep Cheat Engine running after you're done with that value locked at 0.)