Excess neurotransmitters in the synapse are typically removed by these processes:

1. Reuptake into the presynaptic neuron or surrounding glial cells.
2. Enzymatic degradation where enzymes break down neurotransmitters into inactive fragments.
3. Diffusion away from the synaptic cleft.

The process that does NOT occur to excess neurotransmitters in the synapse is "collection by scavenger vesicles left over from the neurotransmitter release," as such vesicles do not exist or function to collect excess neurotransmitters in the synapse. Hence, the correct answer is that excess neurotransmitters are not removed by collection by scavenger vesicles left over from neurotransmitter release.