Are neurotransmitters expelled from the presynaptic cells? Do they tend to destroy acetylcholine?

Two neurons can communicate via the chemical Synapse:
The transmitter (the Presynaptic Neuron) releases the Neurotransmitter of choice (not necessarily AcetylCholine, or " ACh ", there are a LOT of others) into the synaptic cleft. These molecules then trigger the receptors that are sensitive to them.

Acetylcholine is a NeuroTransmitter that is used in the brain in the Acetylcholine pathway :

Acetylcholine is created in the Presynaptic Neuron by combining Choline with Acetate , the latter as part of Acetyl-CoEnzyme A:better known As AcCoA . The enzyme Choline Acetyl Transferase transfers the acetylgroup from AcCoA to Choline.

Apart from its role in the brain pathway, ACh is the transmitter of choice for activating the skeletal muscles . It has to be removed quickly though, as it tends to stick to the receptors when the concentration of Ach in the synaptic fluid is high. Removal is being taken care of by the enzyme AcetylCholineEsterase ( AChE ), which breaks the molecule up in Choline and Acetate (Acetic acid). It works very quickly: 1 enzyme molecule will hydrolyse 25000 molecules of ACh per second...

The Choline is taken up again by the presynaptic neuron, where it will be acetylated again, ready to be released again for a new impulse...