Fixing cracks in a cinder block wall
Short answer is: there is no quick fix with any kind of patching material. The trees will eventually destroy the wall. 

You said the trees are not on your property so you can't remove them. You can however remove any part of them that does come over onto your property including the roots.

The only way you can affect a long term fix is to excavate behind the wall removing the offending roots and heavy soil. Replace it with sand or light gravel to increase the drainage and reduce the attraction of organic growth. You can rent a mini-excavator for a few hundred dollars and have the sand delivered for around $100 a dump truck full. Depending on the size of the wall you could get out of it for less than $1000.

After excavating, and before backfilling you need to straighten the wall and insert 1/2" re-rod into a core every couple feet and fill it with light concrete mix. This will help to stop the cracking from happening again in the near future and support the wall for backfilling. Let it cure for a week before backfilling. People falsely believe that the mortar between concrete blocks gives them strength, the truth is the mortar is only there to seal the joints. A poorly reinforced block wall is easily pushed in. Sounds like whoever built the wall didn't understand this.

Good luck!