It is straightforward to subtract a point source at the phase center
in the 
data: one simply subtract a real number (the source
flux) from all the visibilities.
The task UV_SUBTRACT subtracts a time-averaged continuum
table from a spectral line table (this assumes that the continuum
and the line have been observed simultaneously), providing a new
table with the line emission. Note that if the source is too
complex, the time averaging (needed to avoid increasing the noise
level in the resulting table), may affect the structure of the
subtracted continuum image.
If the continuum data was not observed in the same session, or is
known only from other sources, one may build a table of the
continuum using the task UV_MODEL. This tasks computes that
table from a model image or data cube; it computes the corresponding
visibilities at coordinates taken from a reference table
(e.g. the table out of which one wants to subtract the continuum
model).