Does an action potential abolish an excitatory postsynaptic potential?
While model neurons like the leaky integrate and fire may use a simplification in which the neuron forgets all previous information when it emits a spike, in a biological neuron, the synapse and the soma are relatively electrically isolated from each other, so the voltage activity of the action potential does not make the synapse "forget" the EPSP.  Although see back propagating action potentials for a mechanism for somatic action potentials to affect dendritic processing.  (http://www.ncbi.nlm.nih.gov/pubmed/8107777)