Ubiquitin is typically attached covalently to lysine. To abolish ubiquitylation of your protein of interest, you need to identify then mutate the lysine residues involved. The IBIS server gives both known and predicted interactions so I would first check whether the reported GRB2-ubiquitin interaction is already known or whether it's a prediction. If the latter, the first thing I would do is test whether the prediction is true i.e. test if GRB2 is indeed ubiquitylated.

It is a complex biology process and it would be useful for you to visit http://www.creative-proteomics.com/about-us.htm