Q:

Remove only a trailing semi-colon

At first I tried:
update myTable set myColumn = replace(myColumn,';','')

However the data might have a valid ; in the middle of the data. Before:  
myColumn 
not;trailing
not;trailing and trailing; 
trailing;

After:
myColumn 
not;trailing
not;trailing and trailing
trailing

A:

You can make good use of left (or substring) and len in this case:
update myTable
set myColumn = left(myColumn, len(myColumn) - 1)
where myColumn like '%;'

