Whatever method you use to find the line numbers they can be used as a range for a substitute:
:xx,yy s/^/# /
Move to the first line and set a mark with: ma (where "a" is any letter you choose) Move to the last line and issue the following ex:
:'a,. s/^/# /
More elaborate sequences of pure old ex commands can also be used to create boxes around C/C++ blocks; but they are really horrid to type every time so they have to be mapped to some key sequence and entails consistently using the same register every time.
For example:
map gC :'a,. s/^/ */^M:. s/\(.*\)/\1^V^V^M **************\//^M:'a s/\(.*\)/\/**************^V^V^M\1/^M
maps the sequence gC to a macro which wraps a block of text, from the current line back to the line marked by the "a" in a C style comment like:
/************************ * * .... * .... ************************/