Not in an arbitrary way, no. A councillor has a right to have matters added to the agenda. But the clerk can refuse to accept an item for certain specific reasons. An agenda item must not be unlawful (the council can only discuss matters where it has a competence). An agenda item must be clear in its meaning. The clerk can refuse to accept an unclear item and ask the councillor to clarify it, although often a clerk will help to clarify. There is frequently a deadline (in standing orders) of so many days before a meeting for submission of agenda items, for obvious practical reasons. But in this case, an item that misses the deadline should be included in the next meeting.