Is it possible to remove translation from a composit matrix so that I'm left with a rotation matrix?

Yes, the resulting translation can simply be removed: Substitute the belonging matrix elements with zeros. Whether or not the remaining matrix represents a pure rotation depends on how the matrix was composed. If the composition was done using translations and rotations only, then the remaining matrix will represent a rotation.

For detailed informations and what to do if e.g. also scaling plays a role you may look for the keyword "matrix decomposition".