Hi there. We had that situation (and still do to an extent) at my workplace. Policy changed and PCAs not allowed to administer medications any more: this was a job to be done by EN and/or RN only. This helped. Also ensure they know what you expect: all PRNs are to be checked prior to administration; every time this does not happen, follow up verbally with PCA for a reason, then write incident form: each and every time. Have a meeting with your manager and let them know what is happening and how they wish you to handle it. If PCAs aren't listening to you, it sounds as though management will have to have a staff meeting and review PCA job duties and policy regarding clinical care.
It can get very dodgy when PCAs help out with clinical care which is why a clear division of duties has to be made between trained staff and carers. PCAs are very valuable for the very hard job of doing the 'front line' care of the elderly and deserve respect and hopefully would appreciate hearing that things are done a certain way by trained staff for the benefit of the residents; eg the consequences of PRNs given incorrectly can be extremely harmful which is why it must be reviewed by EN/RN. This is respectful also for the EN/RN in charge; after all, if anything goes wrong, it will be their fault alone and the blame will rest on them for not following up and being told these things. Change is very hard to adjust to but stick at it. Good luck.