It is straightforward to subtract a point source at the phase center in the uv data: one simply subtract a real number (the source flux) from all the visibilities.
The task UV_SUBTRACT subtracts a time-averaged continuum uv 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 uv 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 uv coordinates taken from a reference uv table (e.g. the table out of which one wants to subtract the continuum model).