After a very long research on MSDN, I've found the answer at a proper place (codeProject)... Which should have actually been the first place to look :P
Based on "
change font style in ssrs for some values" + trial and error I've performed the following steps:
1) Selected the desired part of the text.
2) Right clicked and choosing properties
3) Font
4) Expression button from effects
5) Applied =IIF(Fields!TT_ID.Value = 1, "Default", "LineThrough")