I was under the impression no one but the priest given permission from the Vatican could do such a thing.. hopefully someone here knows for sure.

According to Canon Law, Catholics in good standing have a right to receive the Sacraments.  Now, if it's a question of judging whether a convert should be admitted to the Sacraments (having become properly disposed and catechised, etc.), then that's typically at the judgment of the priest, and the priest may have deferred to the judgment of the catechist, for practical reasons (being away and flying in on the weekends), but a "catechist" certainly has no official status or right to prevent the reception of the Sacraments, except through delegation from a priest.  You need to provide more information, OP.

Priests can (and are required to) withhold some Sacraments if their reception by a certain party would cause scandal, but it's a phenomenon of the modern Traditional movement for priests to effectively declare certain persons heretics or schismatics or as scandalous due to specific theological positions.  That I consider to be wrong unless the position taken directly contradicts defined Church teaching (e.g. if someone were denying the Holy Trinity).  If someone, for instance, were to approach the Communion rail immodestly dressed, the priest can and should not give them Holy Communion ... but even that should be limited to the specific incident and not be a generic "excommunication".