There are many ways to do this, but since we are talking about this function then the solution would be, (as Arkadeep said in his comment to your question), to add the delete commands for other tables too. Which would delete the data from other tables also, once it has finished deleting from one table.
SqlCommand command = new SqlCommand("DELETE FROM ads WHERE UID = @UID", connection);
SqlCommand command = new SqlCommand("DELETE FROM CVs WHERE UID = @UID", connection);
SqlCommand command = new SqlCommand("DELETE FROM FavComp WHERE UID = @UID", connection);
SqlCommand command = new SqlCommand("DELETE FROM FavJob WHERE UID = @UID", connection);
...
This way, these commands would delete the record for the user with that ID in other tables also.
Another way would be to implement
referential integrity[
^] and store the UserInfo as Master table. So that the child would hold the data for those records (only!) which exist in the master table. If master deletes or updates on record, other tables update themselves based on the new data.
This method would take a bit of time as you may need to learn about the concept first.